:sparkles: Implement Page bundling and image handling

This commit is not the smallest in Hugo's history.

Some hightlights include:

* Page bundles (for complete articles, keeping images and content together etc.).
* Bundled images can be processed in as many versions/sizes as you need with the three methods `Resize`, `Fill` and `Fit`.
* Processed images are cached inside `resources/_gen/images` (default) in your project.
* Symbolic links (both files and dirs) are now allowed anywhere inside /content
* A new table based build summary
* The "Total in nn ms" now reports the total including the handling of the files inside /static. So if it now reports more than you're used to, it is just **more real** and probably faster than before (see below).

A site building  benchmark run compared to `v0.31.1` shows that this should be slightly faster and use less memory:

```bash
▶ ./benchSite.sh "TOML,num_langs=.*,num_root_sections=5,num_pages=(500|1000),tags_per_page=5,shortcodes,render"

benchmark                                                                                                         old ns/op     new ns/op     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      101785785     78067944      -23.30%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     185481057     149159919     -19.58%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      103149918     85679409      -16.94%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     203515478     169208775     -16.86%

benchmark                                                                                                         old allocs     new allocs     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      532464         391539         -26.47%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1056549        772702         -26.87%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      555974         406630         -26.86%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     1086545        789922         -27.30%

benchmark                                                                                                         old bytes     new bytes     delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      53243246      43598155      -18.12%
BenchmarkSiteBuilding/TOML,num_langs=1,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     105811617     86087116      -18.64%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=500,tags_per_page=5,shortcodes,render-4      54558852      44545097      -18.35%
BenchmarkSiteBuilding/TOML,num_langs=3,num_root_sections=5,num_pages=1000,tags_per_page=5,shortcodes,render-4     106903858     86978413      -18.64%
```

Fixes #3651
Closes #3158
Fixes #1014
Closes #2021
Fixes #1240
Updates #3757

1 files changed, 13 insertions, 53 deletions

diff --git a/source/filesystem_test.go b/source/filesystem_test.go
index 90512ce3f..25ce0268f 100644
--- a/source/filesystem_test.go
+++ b/source/filesystem_test.go
@@ -14,11 +14,13 @@
 package source
 
 import (
-	"bytes"
-	"path/filepath"
+	"os"
 	"runtime"
-	"strings"
 	"testing"
+
+	"github.com/gohugoio/hugo/hugofs"
+
+	"github.com/spf13/viper"
 )
 
 func TestEmptySourceFilesystem(t *testing.T) {
@@ -37,54 +39,6 @@ type TestPath struct {
 	dir      string
 }
 
-func TestAddFile(t *testing.T) {
-	ss := newTestSourceSpec()
-	tests := platformPaths
-	for _, test := range tests {
-		base := platformBase
-		srcDefault := ss.NewFilesystem("")
-		srcWithBase := ss.NewFilesystem(base)
-
-		for _, src := range []*Filesystem{srcDefault, srcWithBase} {
-
-			p := test.filename
-			if !filepath.IsAbs(test.filename) {
-				p = filepath.Join(src.Base, test.filename)
-			}
-
-			if err := src.add(p, bytes.NewReader([]byte(test.content))); err != nil {
-				if err.Error() == "source: missing base directory" {
-					continue
-				}
-				t.Fatalf("%s add returned an error: %s", p, err)
-			}
-
-			if len(src.Files()) != 1 {
-				t.Fatalf("%s Files() should return 1 file", p)
-			}
-
-			f := src.Files()[0]
-			if f.LogicalName() != test.logical {
-				t.Errorf("Filename (Base: %q) expected: %q, got: %q", src.Base, test.logical, f.LogicalName())
-			}
-
-			b := new(bytes.Buffer)
-			b.ReadFrom(f.Contents)
-			if b.String() != test.content {
-				t.Errorf("File (Base: %q) contents should be %q, got: %q", src.Base, test.content, b.String())
-			}
-
-			if f.Section() != test.section {
-				t.Errorf("File section (Base: %q) expected: %q, got: %q", src.Base, test.section, f.Section())
-			}
-
-			if f.Dir() != test.dir {
-				t.Errorf("Dir path (Base: %q) expected: %q, got: %q", src.Base, test.dir, f.Dir())
-			}
-		}
-	}
-}
-
 func TestUnicodeNorm(t *testing.T) {
 	if runtime.GOOS != "darwin" {
 		// Normalization code is only for Mac OS, since it is not necessary for other OSes.
@@ -100,10 +54,11 @@ func TestUnicodeNorm(t *testing.T) {
 	}
 
 	ss := newTestSourceSpec()
+	var fi os.FileInfo
 
 	for _, path := range paths {
-		src := ss.NewFilesystem("")
-		_ = src.add(path.NFD, strings.NewReader(""))
+		src := ss.NewFilesystem("base")
+		_ = src.add(path.NFD, fi)
 		f := src.Files()[0]
 		if f.BaseFileName() != path.NFC {
 			t.Fatalf("file name in NFD form should be normalized (%s)", path.NFC)
@@ -111,3 +66,8 @@ func TestUnicodeNorm(t *testing.T) {
 	}
 
 }
+
+func newTestSourceSpec() SourceSpec {
+	v := viper.New()
+	return SourceSpec{Fs: hugofs.NewMem(v), Cfg: v}
+}