✨ 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

hugolib/fileInfo.go

@@ -0,0 +1,109 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"os"
+	"strings"
+
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/source"
+)
+
+// fileInfo implements the File and ReadableFile interface.
+var (
+	_ source.File         = (*fileInfo)(nil)
+	_ source.ReadableFile = (*fileInfo)(nil)
+)
+
+type fileInfo struct {
+	bundleTp bundleDirType
+	source.ReadableFile
+	overriddenLang string
+}
+
+func (fi *fileInfo) Lang() string {
+	if fi.overriddenLang != "" {
+		return fi.overriddenLang
+	}
+	return fi.ReadableFile.Lang()
+}
+
+func (fi *fileInfo) isOwner() bool {
+	return fi.bundleTp > bundleNot
+}
+
+func isContentFile(filename string) bool {
+	return contentFileExtensionsSet[strings.TrimPrefix(helpers.Ext(filename), ".")]
+}
+
+func (fi *fileInfo) isContentFile() bool {
+	return contentFileExtensionsSet[fi.Ext()]
+}
+
+func newFileInfo(sp *source.SourceSpec, baseDir, filename string, fi os.FileInfo, tp bundleDirType) *fileInfo {
+
+	baseFi := sp.NewFileInfo(baseDir, filename, fi)
+	f := &fileInfo{
+		bundleTp:     tp,
+		ReadableFile: baseFi,
+	}
+
+	return f
+
+}
+
+type bundleDirType int
+
+const (
+	bundleNot bundleDirType = iota
+
+	// All from here are bundles in one form or another.
+	bundleLeaf
+	bundleBranch
+)
+
+// Returns the given file's name's bundle type and whether it is a content
+// file or not.
+func classifyBundledFile(name string) (bundleDirType, bool) {
+	if !isContentFile(name) {
+		return bundleNot, false
+	}
+	if strings.HasPrefix(name, "_index.") {
+		return bundleBranch, true
+	}
+
+	if strings.HasPrefix(name, "index.") {
+		return bundleLeaf, true
+	}
+
+	return bundleNot, true
+}
+
+func (b bundleDirType) String() string {
+	switch b {
+	case bundleNot:
+		return "Not a bundle"
+	case bundleLeaf:
+		return "Regular bundle"
+	case bundleBranch:
+		return "Branch bundle"
+	}
+
+	return ""
+}
+
+func (b bundleDirType) isBundle() bool {
+	return b > bundleNot
+}