Add support for a content dir set per language

A sample config:

```toml
defaultContentLanguage = "en"
defaultContentLanguageInSubdir = true

[Languages]
[Languages.en]
weight = 10
title = "In English"
languageName = "English"
contentDir = "content/english"

[Languages.nn]
weight = 20
title = "På Norsk"
languageName = "Norsk"
contentDir = "content/norwegian"
```

The value of `contentDir` can be any valid path, even absolute path references. The only restriction is that the content dirs cannot overlap.

The content files will be assigned a language by

1. The placement: `content/norwegian/post/my-post.md` will be read as Norwegian content.
2. The filename: `content/english/post/my-post.nn.md` will be read as Norwegian even if it lives in the English content folder.

The content directories will be merged into a big virtual filesystem with one simple rule: The most specific language file will win.
This means that if both `content/norwegian/post/my-post.md` and `content/english/post/my-post.nn.md` exists, they will be considered duplicates and the version inside `content/norwegian` will win.

Note that translations will be automatically assigned by Hugo by the content file's relative placement, so `content/norwegian/post/my-post.md` will be a translation of `content/english/post/my-post.md`.

If this does not work for you, you can connect the translations together by setting a `translationKey` in the content files' front matter.

Fixes #4523
Fixes #4552
Fixes #4553

tpl/tplimpl/template_funcs_test.go

@@ -65,6 +65,7 @@ func TestTemplateFuncsExamples(t *testing.T) {
 
 	v.Set("workingDir", workingDir)
 	v.Set("multilingual", true)
+	v.Set("contentDir", "content")
 	v.Set("baseURL", "http://mysite.com/hugo/")
 	v.Set("CurrentContentLanguage", helpers.NewLanguage("en", v))
 
@@ -125,7 +126,10 @@ func TestPartialCached(t *testing.T) {
 	var data struct {
 	}
 
-	config := newDepsConfig(viper.New())
+	v := viper.New()
+	v.Set("contentDir", "content")
+
+	config := newDepsConfig(v)
 
 	config.WithTemplate = func(templ tpl.TemplateHandler) error {
 		err := templ.AddTemplate("partials/"+name, partial)