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/transform/transform_test.go

@@ -32,7 +32,9 @@ type tstNoStringer struct{}
 func TestEmojify(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -60,7 +62,9 @@ func TestEmojify(t *testing.T) {
 func TestHighlight(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -90,7 +94,9 @@ func TestHighlight(t *testing.T) {
 func TestHTMLEscape(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -118,7 +124,9 @@ func TestHTMLEscape(t *testing.T) {
 func TestHTMLUnescape(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -146,7 +154,9 @@ func TestHTMLUnescape(t *testing.T) {
 func TestMarkdownify(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -176,7 +186,9 @@ func TestMarkdownifyBlocksOfText(t *testing.T) {
 
 	assert := require.New(t)
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	text := `
 #First 
@@ -201,7 +213,9 @@ And then some.
 func TestPlainify(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}