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, 35 insertions, 70 deletions

diff --git a/helpers/path.go b/helpers/path.go
index 7ac9208bf..76f13d653 100644
--- a/helpers/path.go
+++ b/helpers/path.go
@@ -20,6 +20,7 @@ import (
 	"os"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strings"
 	"unicode"
 
@@ -31,9 +32,6 @@ import (
 var (
 	// ErrThemeUndefined is returned when a theme has not be defined by the user.
 	ErrThemeUndefined = errors.New("no theme set")
-
-	// ErrWalkRootTooShort is returned when the root specified for a file walk is shorter than 4 characters.
-	ErrPathTooShort = errors.New("file path is too short")
 )
 
 // filepathPathBridge is a bridge for common functionality in filepath vs path
@@ -86,7 +84,7 @@ func (p *PathSpec) MakePath(s string) string {
 
 // MakePathSanitized creates a Unicode-sanitized string, with the spaces replaced
 func (p *PathSpec) MakePathSanitized(s string) string {
-	if p.disablePathToLower {
+	if p.DisablePathToLower {
 		return p.MakePath(s)
 	}
 	return strings.ToLower(p.MakePath(s))
@@ -129,7 +127,7 @@ func (p *PathSpec) UnicodeSanitize(s string) string {
 
 	var result string
 
-	if p.removePathAccents {
+	if p.RemovePathAccents {
 		// remove accents - see https://blog.golang.org/normalization
 		t := transform.Chain(norm.NFD, transform.RemoveFunc(isMn), norm.NFC)
 		result, _, _ = transform.String(t, string(target))
@@ -151,32 +149,19 @@ func ReplaceExtension(path string, newExt string) string {
 	return f + "." + newExt
 }
 
-// AbsPathify creates an absolute path if given a relative path. If already
-// absolute, the path is just cleaned.
-func (p *PathSpec) AbsPathify(inPath string) string {
-	return AbsPathify(p.workingDir, inPath)
-}
-
-// AbsPathify creates an absolute path if given a working dir and arelative path.
-// If already absolute, the path is just cleaned.
-func AbsPathify(workingDir, inPath string) string {
-	if filepath.IsAbs(inPath) {
-		return filepath.Clean(inPath)
+// GetFirstThemeDir gets the root directory of the first theme, if there is one.
+// If there is no theme, returns the empty string.
+func (p *PathSpec) GetFirstThemeDir() string {
+	if p.ThemeSet() {
+		return p.AbsPathify(filepath.Join(p.ThemesDir, p.Themes()[0]))
 	}
-	return filepath.Join(workingDir, inPath)
-}
-
-// GetLayoutDirPath returns the absolute path to the layout file dir
-// for the current Hugo project.
-func (p *PathSpec) GetLayoutDirPath() string {
-	return p.AbsPathify(p.layoutDir)
+	return ""
 }
 
-// GetThemeDir gets the root directory of the current theme, if there is one.
-// If there is no theme, returns the empty string.
-func (p *PathSpec) GetThemeDir() string {
+// GetThemesDir gets the absolute root theme dir path.
+func (p *PathSpec) GetThemesDir() string {
 	if p.ThemeSet() {
-		return p.AbsPathify(filepath.Join(p.themesDir, p.theme))
+		return p.AbsPathify(p.ThemesDir)
 	}
 	return ""
 }
@@ -185,50 +170,11 @@ func (p *PathSpec) GetThemeDir() string {
 // If there is no theme, returns the empty string.
 func (p *PathSpec) GetRelativeThemeDir() string {
 	if p.ThemeSet() {
-		return strings.TrimPrefix(filepath.Join(p.themesDir, p.theme), FilePathSeparator)
+		return strings.TrimPrefix(filepath.Join(p.ThemesDir, p.Themes()[0]), FilePathSeparator)
 	}
 	return ""
 }
 
-// GetThemeStaticDirPath returns the theme's static dir path if theme is set.
-// If theme is set and the static dir doesn't exist, an error is returned.
-func (p *PathSpec) GetThemeStaticDirPath() (string, error) {
-	return p.getThemeDirPath("static")
-}
-
-// GetThemeDataDirPath returns the theme's data dir path if theme is set.
-// If theme is set and the data dir doesn't exist, an error is returned.
-func (p *PathSpec) GetThemeDataDirPath() (string, error) {
-	return p.getThemeDirPath("data")
-}
-
-// GetThemeI18nDirPath returns the theme's i18n dir path if theme is set.
-// If theme is set and the i18n dir doesn't exist, an error is returned.
-func (p *PathSpec) GetThemeI18nDirPath() (string, error) {
-	return p.getThemeDirPath("i18n")
-}
-
-func (p *PathSpec) getThemeDirPath(path string) (string, error) {
-	if !p.ThemeSet() {
-		return "", ErrThemeUndefined
-	}
-
-	themeDir := filepath.Join(p.GetThemeDir(), path)
-	if _, err := p.Fs.Source.Stat(themeDir); os.IsNotExist(err) {
-		return "", fmt.Errorf("Unable to find %s directory for theme %s in %s", path, p.theme, themeDir)
-	}
-
-	return themeDir, nil
-}
-
-// GetThemesDirPath gets the static files directory of the current theme, if there is one.
-// Ignores underlying errors.
-// TODO(bep) Candidate for deprecation?
-func (p *PathSpec) GetThemesDirPath() string {
-	dir, _ := p.getThemeDirPath("static")
-	return dir
-}
-
 func makePathRelative(inPath string, possibleDirectories ...string) (string, error) {
 
 	for _, currentPath := range possibleDirectories {
@@ -445,8 +391,8 @@ func FindCWD() (string, error) {
 func SymbolicWalk(fs afero.Fs, root string, walker filepath.WalkFunc) error {
 
 	// Sanity check
-	if len(root) < 4 {
-		return ErrPathTooShort
+	if root != "" && len(root) < 4 {
+		return errors.New("Path is too short")
 	}
 
 	// Handle the root first
@@ -464,7 +410,10 @@ func SymbolicWalk(fs afero.Fs, root string, walker filepath.WalkFunc) error {
 		return err
 	}
 
-	rootContent, err := afero.ReadDir(fs, root)
+	// Some of Hugo's filesystems represents an ordered root folder, i.e. project first, then theme folders.
+	// Make sure that order is preserved. afero.Walk will sort the directories down in the file tree,
+	// but we don't care about that.
+	rootContent, err := readDir(fs, root, false)
 
 	if err != nil {
 		return walker(root, nil, err)
@@ -480,6 +429,22 @@ func SymbolicWalk(fs afero.Fs, root string, walker filepath.WalkFunc) error {
 
 }
 
+func readDir(fs afero.Fs, dirname string, doSort bool) ([]os.FileInfo, error) {
+	f, err := fs.Open(dirname)
+	if err != nil {
+		return nil, err
+	}
+	list, err := f.Readdir(-1)
+	f.Close()
+	if err != nil {
+		return nil, err
+	}
+	if doSort {
+		sort.Slice(list, func(i, j int) bool { return list[i].Name() < list[j].Name() })
+	}
+	return list, nil
+}
+
 func getRealFileInfo(fs afero.Fs, path string) (os.FileInfo, string, error) {
 	fileInfo, err := LstatIfPossible(fs, path)
 	realPath := path