Add support for theme composition and inheritance

This commit adds support for theme composition and inheritance in Hugo.

With this, it helps thinking about a theme as a set of ordered components:

```toml
theme = ["my-shortcodes", "base-theme", "hyde"]
```

The theme definition example above in `config.toml` creates a theme with the 3 components with presedence from left to right.

So, Hugo will, for any given file, data entry etc., look first in the project, and then in `my-shortcode`, `base-theme` and lastly `hyde`.

Hugo uses two different algorithms to merge the filesystems, depending on the file type:

* For `i18n` and `data` files, Hugo merges deeply using the translation id and data key inside the files.
* For `static`, `layouts` (templates) and `archetypes` files, these are merged on file level. So the left-most file will be chosen.

The name used in the `theme` definition above must match a folder in `/your-site/themes`, e.g. `/your-site/themes/my-shortcodes`. There are  plans to improve on this and get a URL scheme so this can be resolved automatically.

Also note that a component that is part of a theme can have its own configuration file, e.g. `config.toml`. There are currently some restrictions to what a theme component can configure:

* `params` (global and per language)
* `menu` (global and per language)
* `outputformats` and `mediatypes`

The same rules apply here: The left-most param/menu etc. with the same ID will win. There are some hidden and experimental namespace support in the above, which we will work to improve in the future, but theme authors are encouraged to create their own namespaces to avoid naming conflicts.

A final note: Themes/components can also have a `theme` definition in their `config.toml` and similar, which is the "inheritance" part of this commit's title. This is currently not supported by the Hugo theme site. We will have to wait for some "auto dependency" feature to be implemented for that to happen, but this can be a powerful feature if you want to create your own theme-variant based on others.

Fixes #4460
Fixes #4450

1 files changed, 37 insertions, 43 deletions

diff --git a/output/layout_test.go b/output/layout_test.go
index 3c7fde41a..4b958e9ff 100644
--- a/output/layout_test.go
+++ b/output/layout_test.go
@@ -57,62 +57,61 @@ func TestLayout(t *testing.T) {
 	for _, this := range []struct {
 		name           string
 		d              LayoutDescriptor
-		hasTheme       bool
 		layoutOverride string
 		tp             Format
 		expect         []string
 		expectCount    int
 	}{
-		{"Home", LayoutDescriptor{Kind: "home"}, true, "", ampType,
-			[]string{"index.amp.html", "theme/index.amp.html", "home.amp.html", "theme/home.amp.html", "list.amp.html", "theme/list.amp.html", "index.html", "theme/index.html", "home.html", "theme/home.html", "list.html", "theme/list.html", "_default/index.amp.html"}, 24},
-		{"Home, HTML", LayoutDescriptor{Kind: "home"}, true, "", htmlFormat,
+		{"Home", LayoutDescriptor{Kind: "home"}, "", ampType,
+			[]string{"index.amp.html", "home.amp.html", "list.amp.html", "index.html", "home.html", "list.html", "_default/index.amp.html"}, 12},
+		{"Home, HTML", LayoutDescriptor{Kind: "home"}, "", htmlFormat,
 			// We will eventually get to index.html. This looks stuttery, but makes the lookup logic easy to understand.
-			[]string{"index.html.html", "theme/index.html.html", "home.html.html"}, 24},
-		{"Home, french language", LayoutDescriptor{Kind: "home", Lang: "fr"}, true, "", ampType,
-			[]string{"index.fr.amp.html", "theme/index.fr.amp.html"},
-			48},
-		{"Home, no ext or delim", LayoutDescriptor{Kind: "home"}, true, "", noExtDelimFormat,
-			[]string{"index.nem", "theme/index.nem", "home.nem", "theme/home.nem", "list.nem"}, 12},
-		{"Home, no ext", LayoutDescriptor{Kind: "home"}, true, "", noExt,
-			[]string{"index.nex", "theme/index.nex", "home.nex", "theme/home.nex", "list.nex"}, 12},
-		{"Page, no ext or delim", LayoutDescriptor{Kind: "page"}, true, "", noExtDelimFormat,
-			[]string{"_default/single.nem", "theme/_default/single.nem"}, 2},
-		{"Section", LayoutDescriptor{Kind: "section", Section: "sect1"}, false, "", ampType,
+			[]string{"index.html.html", "home.html.html"}, 12},
+		{"Home, french language", LayoutDescriptor{Kind: "home", Lang: "fr"}, "", ampType,
+			[]string{"index.fr.amp.html"},
+			24},
+		{"Home, no ext or delim", LayoutDescriptor{Kind: "home"}, "", noExtDelimFormat,
+			[]string{"index.nem", "home.nem", "list.nem"}, 6},
+		{"Home, no ext", LayoutDescriptor{Kind: "home"}, "", noExt,
+			[]string{"index.nex", "home.nex", "list.nex"}, 6},
+		{"Page, no ext or delim", LayoutDescriptor{Kind: "page"}, "", noExtDelimFormat,
+			[]string{"_default/single.nem"}, 1},
+		{"Section", LayoutDescriptor{Kind: "section", Section: "sect1"}, "", ampType,
 			[]string{"sect1/sect1.amp.html", "sect1/section.amp.html", "sect1/list.amp.html", "sect1/sect1.html", "sect1/section.html", "sect1/list.html", "section/sect1.amp.html", "section/section.amp.html"}, 18},
-		{"Section with layout", LayoutDescriptor{Kind: "section", Section: "sect1", Layout: "mylayout"}, false, "", ampType,
+		{"Section with layout", LayoutDescriptor{Kind: "section", Section: "sect1", Layout: "mylayout"}, "", ampType,
 			[]string{"sect1/mylayout.amp.html", "sect1/sect1.amp.html", "sect1/section.amp.html", "sect1/list.amp.html", "sect1/mylayout.html", "sect1/sect1.html"}, 24},
-		{"Taxonomy", LayoutDescriptor{Kind: "taxonomy", Section: "tag"}, false, "", ampType,
+		{"Taxonomy", LayoutDescriptor{Kind: "taxonomy", Section: "tag"}, "", ampType,
 			[]string{"taxonomy/tag.amp.html", "taxonomy/taxonomy.amp.html", "taxonomy/list.amp.html", "taxonomy/tag.html", "taxonomy/taxonomy.html"}, 18},
-		{"Taxonomy term", LayoutDescriptor{Kind: "taxonomyTerm", Section: "categories"}, false, "", ampType,
+		{"Taxonomy term", LayoutDescriptor{Kind: "taxonomyTerm", Section: "categories"}, "", ampType,
 			[]string{"taxonomy/categories.terms.amp.html", "taxonomy/terms.amp.html", "taxonomy/list.amp.html", "taxonomy/categories.terms.html", "taxonomy/terms.html"}, 18},
-		{"Page", LayoutDescriptor{Kind: "page"}, true, "", ampType,
-			[]string{"_default/single.amp.html", "theme/_default/single.amp.html", "_default/single.html", "theme/_default/single.html"}, 4},
-		{"Page with layout", LayoutDescriptor{Kind: "page", Layout: "mylayout"}, false, "", ampType,
+		{"Page", LayoutDescriptor{Kind: "page"}, "", ampType,
+			[]string{"_default/single.amp.html", "_default/single.html"}, 2},
+		{"Page with layout", LayoutDescriptor{Kind: "page", Layout: "mylayout"}, "", ampType,
 			[]string{"_default/mylayout.amp.html", "_default/single.amp.html", "_default/mylayout.html", "_default/single.html"}, 4},
-		{"Page with layout and type", LayoutDescriptor{Kind: "page", Layout: "mylayout", Type: "myttype"}, false, "", ampType,
+		{"Page with layout and type", LayoutDescriptor{Kind: "page", Layout: "mylayout", Type: "myttype"}, "", ampType,
 			[]string{"myttype/mylayout.amp.html", "myttype/single.amp.html", "myttype/mylayout.html"}, 8},
-		{"Page with layout and type with subtype", LayoutDescriptor{Kind: "page", Layout: "mylayout", Type: "myttype/mysubtype"}, false, "", ampType,
+		{"Page with layout and type with subtype", LayoutDescriptor{Kind: "page", Layout: "mylayout", Type: "myttype/mysubtype"}, "", ampType,
 			[]string{"myttype/mysubtype/mylayout.amp.html", "myttype/mysubtype/single.amp.html", "myttype/mysubtype/mylayout.html"}, 8},
 		// RSS
-		{"RSS Home with theme", LayoutDescriptor{Kind: "home"}, true, "", RSSFormat,
-			[]string{"index.rss.xml", "theme/index.rss.xml", "home.rss.xml", "theme/home.rss.xml", "rss.xml"}, 29},
-		{"RSS Section", LayoutDescriptor{Kind: "section", Section: "sect1"}, false, "", RSSFormat,
+		{"RSS Home", LayoutDescriptor{Kind: "home"}, "", RSSFormat,
+			[]string{"index.rss.xml", "home.rss.xml", "rss.xml"}, 15},
+		{"RSS Section", LayoutDescriptor{Kind: "section", Section: "sect1"}, "", RSSFormat,
 			[]string{"sect1/sect1.rss.xml", "sect1/section.rss.xml", "sect1/rss.xml", "sect1/list.rss.xml", "sect1/sect1.xml", "sect1/section.xml"}, 22},
-		{"RSS Taxonomy", LayoutDescriptor{Kind: "taxonomy", Section: "tag"}, false, "", RSSFormat,
+		{"RSS Taxonomy", LayoutDescriptor{Kind: "taxonomy", Section: "tag"}, "", RSSFormat,
 			[]string{"taxonomy/tag.rss.xml", "taxonomy/taxonomy.rss.xml", "taxonomy/rss.xml", "taxonomy/list.rss.xml", "taxonomy/tag.xml", "taxonomy/taxonomy.xml"}, 22},
-		{"RSS Taxonomy term", LayoutDescriptor{Kind: "taxonomyTerm", Section: "tag"}, false, "", RSSFormat,
+		{"RSS Taxonomy term", LayoutDescriptor{Kind: "taxonomyTerm", Section: "tag"}, "", RSSFormat,
 			[]string{"taxonomy/tag.terms.rss.xml", "taxonomy/terms.rss.xml", "taxonomy/rss.xml", "taxonomy/list.rss.xml", "taxonomy/tag.terms.xml"}, 22},
-		{"Home plain text", LayoutDescriptor{Kind: "home"}, true, "", JSONFormat,
-			[]string{"_text/index.json.json", "_text/theme/index.json.json", "_text/home.json.json", "_text/theme/home.json.json"}, 24},
-		{"Page plain text", LayoutDescriptor{Kind: "page"}, true, "", JSONFormat,
-			[]string{"_text/_default/single.json.json", "_text/theme/_default/single.json.json", "_text/_default/single.json", "_text/theme/_default/single.json"}, 4},
-		{"Reserved section, shortcodes", LayoutDescriptor{Kind: "section", Section: "shortcodes", Type: "shortcodes"}, true, "", ampType,
-			[]string{"section/shortcodes.amp.html", "theme/section/shortcodes.amp.html"}, 24},
-		{"Reserved section, partials", LayoutDescriptor{Kind: "section", Section: "partials", Type: "partials"}, true, "", ampType,
-			[]string{"section/partials.amp.html", "theme/section/partials.amp.html"}, 24},
+		{"Home plain text", LayoutDescriptor{Kind: "home"}, "", JSONFormat,
+			[]string{"_text/index.json.json", "_text/home.json.json"}, 12},
+		{"Page plain text", LayoutDescriptor{Kind: "page"}, "", JSONFormat,
+			[]string{"_text/_default/single.json.json", "_text/_default/single.json"}, 2},
+		{"Reserved section, shortcodes", LayoutDescriptor{Kind: "section", Section: "shortcodes", Type: "shortcodes"}, "", ampType,
+			[]string{"section/shortcodes.amp.html"}, 12},
+		{"Reserved section, partials", LayoutDescriptor{Kind: "section", Section: "partials", Type: "partials"}, "", ampType,
+			[]string{"section/partials.amp.html"}, 12},
 	} {
 		t.Run(this.name, func(t *testing.T) {
-			l := NewLayoutHandler(this.hasTheme)
+			l := NewLayoutHandler()
 
 			layouts, err := l.For(this.d, this.tp)
 
@@ -130,11 +129,6 @@ func TestLayout(t *testing.T) {
 
 			}
 
-			if !this.hasTheme {
-				for _, layout := range layouts {
-					require.NotContains(t, layout, "theme")
-				}
-			}
 		})
 	}
 
@@ -142,7 +136,7 @@ func TestLayout(t *testing.T) {
 
 func BenchmarkLayout(b *testing.B) {
 	descriptor := LayoutDescriptor{Kind: "taxonomyTerm", Section: "categories"}
-	l := NewLayoutHandler(false)
+	l := NewLayoutHandler()
 
 	for i := 0; i < b.N; i++ {
 		layouts, err := l.For(descriptor, HTMLFormat)