: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, 78 insertions, 0 deletions

diff --git a/resource/testhelpers_test.go b/resource/testhelpers_test.go
new file mode 100644
index 000000000..7ab210688
--- /dev/null
+++ b/resource/testhelpers_test.go
@@ -0,0 +1,78 @@
+package resource
+
+import (
+	"path/filepath"
+	"testing"
+
+	"image"
+	"io"
+	"os"
+	"path"
+
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugofs"
+	"github.com/gohugoio/hugo/media"
+	"github.com/spf13/afero"
+	"github.com/spf13/viper"
+	"github.com/stretchr/testify/require"
+)
+
+func newTestResourceSpec(assert *require.Assertions) *Spec {
+	cfg := viper.New()
+	cfg.Set("baseURL", "https://example.com/")
+	cfg.Set("resourceDir", "/res")
+	fs := hugofs.NewMem(cfg)
+
+	s, err := helpers.NewPathSpec(fs, cfg)
+	assert.NoError(err)
+
+	spec, err := NewSpec(s, media.DefaultTypes)
+	assert.NoError(err)
+	return spec
+}
+
+func fetchSunset(assert *require.Assertions) *Image {
+	src, err := os.Open("testdata/sunset.jpg")
+	assert.NoError(err)
+
+	spec := newTestResourceSpec(assert)
+
+	out, err := spec.Fs.Source.Create("/b/sunset.jpg")
+	assert.NoError(err)
+	_, err = io.Copy(out, src)
+	out.Close()
+	src.Close()
+	assert.NoError(err)
+
+	factory := func(s string) string {
+		return path.Join("/a", s)
+	}
+
+	r, err := spec.NewResourceFromFilename(factory, "/public", "/b/sunset.jpg", "sunset.jpg")
+	assert.NoError(err)
+	assert.IsType(&Image{}, r)
+	return r.(*Image)
+
+}
+
+func assertFileCache(assert *require.Assertions, fs *hugofs.Fs, filename string, width, height int) {
+	f, err := fs.Source.Open(filepath.Join("/res/_gen/images", filename))
+	assert.NoError(err)
+	defer f.Close()
+
+	config, _, err := image.DecodeConfig(f)
+	assert.NoError(err)
+
+	assert.Equal(width, config.Width)
+	assert.Equal(height, config.Height)
+}
+
+func writeSource(t testing.TB, fs *hugofs.Fs, filename, content string) {
+	writeToFs(t, fs.Source, filename, content)
+}
+
+func writeToFs(t testing.TB, fs afero.Fs, filename, content string) {
+	if err := afero.WriteFile(fs, filepath.FromSlash(filename), []byte(content), 0755); err != nil {
+		t.Fatalf("Failed to write file: %s", err)
+	}
+}