✨ 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

source/sourceSpec.go

@@ -0,0 +1,117 @@
+// 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 source
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+
+	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugofs"
+	"github.com/spf13/cast"
+)
+
+// SourceSpec abstracts language-specific file creation.
+// TODO(bep) rename to Spec
+type SourceSpec struct {
+	Cfg config.Provider
+	Fs  *hugofs.Fs
+
+	// This is set if the ignoreFiles config is set.
+	ignoreFilesRe []*regexp.Regexp
+
+	Languages              map[string]interface{}
+	DefaultContentLanguage string
+}
+
+// NewSourceSpec initializes SourceSpec using languages from a given configuration.
+func NewSourceSpec(cfg config.Provider, fs *hugofs.Fs) *SourceSpec {
+	defaultLang := cfg.GetString("defaultContentLanguage")
+	languages := cfg.GetStringMap("languages")
+
+	if len(languages) == 0 {
+		l := helpers.NewDefaultLanguage(cfg)
+		languages[l.Lang] = l
+		defaultLang = l.Lang
+	}
+
+	ignoreFiles := cast.ToStringSlice(cfg.Get("ignoreFiles"))
+	var regexps []*regexp.Regexp
+	if len(ignoreFiles) > 0 {
+		for _, ignorePattern := range ignoreFiles {
+			re, err := regexp.Compile(ignorePattern)
+			if err != nil {
+				helpers.DistinctErrorLog.Printf("Invalid regexp %q in ignoreFiles: %s", ignorePattern, err)
+			} else {
+				regexps = append(regexps, re)
+			}
+
+		}
+	}
+
+	return &SourceSpec{ignoreFilesRe: regexps, Cfg: cfg, Fs: fs, Languages: languages, DefaultContentLanguage: defaultLang}
+}
+
+func (s *SourceSpec) IgnoreFile(filename string) bool {
+	base := filepath.Base(filename)
+
+	if len(base) > 0 {
+		first := base[0]
+		last := base[len(base)-1]
+		if first == '.' ||
+			first == '#' ||
+			last == '~' {
+			return true
+		}
+	}
+
+	if len(s.ignoreFilesRe) == 0 {
+		return false
+	}
+
+	for _, re := range s.ignoreFilesRe {
+		if re.MatchString(filename) {
+			return true
+		}
+	}
+
+	return false
+}
+
+func (s *SourceSpec) IsRegularSourceFile(filename string) (bool, error) {
+	fi, err := helpers.LstatIfOs(s.Fs.Source, filename)
+	if err != nil {
+		return false, err
+	}
+
+	if fi.IsDir() {
+		return false, nil
+	}
+
+	if fi.Mode()&os.ModeSymlink == os.ModeSymlink {
+		link, err := filepath.EvalSymlinks(filename)
+		fi, err = helpers.LstatIfOs(s.Fs.Source, link)
+		if err != nil {
+			return false, err
+		}
+
+		if fi.IsDir() {
+			return false, nil
+		}
+	}
+
+	return true, nil
+}