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, 24 insertions, 30 deletions

diff --git a/helpers/general.go b/helpers/general.go
index 5b46520e5..b442b1eb4 100644
--- a/helpers/general.go
+++ b/helpers/general.go
@@ -20,18 +20,20 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"os"
 	"path/filepath"
 	"strings"
 	"sync"
 	"unicode"
 	"unicode/utf8"
 
+	"github.com/gohugoio/hugo/hugofs"
+
 	"github.com/spf13/afero"
 
 	"github.com/jdkato/prose/transform"
 
 	bp "github.com/gohugoio/hugo/bufferpool"
-	"github.com/spf13/cast"
 	jww "github.com/spf13/jwalterweatherman"
 	"github.com/spf13/pflag"
 )
@@ -129,30 +131,6 @@ func ReaderToBytes(lines io.Reader) []byte {
 	return bc
 }
 
-// ToLowerMap makes all the keys in the given map lower cased and will do so
-// recursively.
-// Notes:
-// * This will modify the map given.
-// * Any nested map[interface{}]interface{} will be converted to map[string]interface{}.
-func ToLowerMap(m map[string]interface{}) {
-	for k, v := range m {
-		switch v.(type) {
-		case map[interface{}]interface{}:
-			v = cast.ToStringMap(v)
-			ToLowerMap(v.(map[string]interface{}))
-		case map[string]interface{}:
-			ToLowerMap(v.(map[string]interface{}))
-		}
-
-		lKey := strings.ToLower(k)
-		if k != lKey {
-			delete(m, k)
-			m[lKey] = v
-		}
-
-	}
-}
-
 // ReaderToString is the same as ReaderToBytes, but returns a string.
 func ReaderToString(lines io.Reader) string {
 	if lines == nil {
@@ -255,11 +233,6 @@ func compareStringSlices(a, b []string) bool {
 	return true
 }
 
-// ThemeSet checks whether a theme is in use or not.
-func (p *PathSpec) ThemeSet() bool {
-	return p.theme != ""
-}
-
 // LogPrinter is the common interface of the JWWs loggers.
 type LogPrinter interface {
 	// Println is the only common method that works in all of JWWs loggers.
@@ -477,3 +450,24 @@ func DiffStringSlices(slice1 []string, slice2 []string) []string {
 func DiffStrings(s1, s2 string) []string {
 	return DiffStringSlices(strings.Fields(s1), strings.Fields(s2))
 }
+
+// PrintFs prints the given filesystem to the given writer starting from the given path.
+// This is useful for debugging.
+func PrintFs(fs afero.Fs, path string, w io.Writer) {
+	if fs == nil {
+		return
+	}
+	afero.Walk(fs, path, func(path string, info os.FileInfo, err error) error {
+		if info != nil && !info.IsDir() {
+			s := path
+			if lang, ok := info.(hugofs.LanguageAnnouncer); ok {
+				s = s + "\tLANG: " + lang.Lang()
+			}
+			if fp, ok := info.(hugofs.FilePather); ok {
+				s = s + "\tRF: " + fp.Filename() + "\tBP: " + fp.BaseDir()
+			}
+			fmt.Fprintln(w, "    ", s)
+		}
+		return nil
+	})
+}