hugolib: Refactor/-work the permalink/target path logic

This is a pretty fundamental change in Hugo, but absolutely needed if we should have any hope of getting "multiple outputs" done.

This commit's goal is to say:

* Every file target path is created by `createTargetPath`, i.e. one function for all.
* That function takes every page and site parameter into account, to avoid fragile string parsing to uglify etc. later on.
* The path creation logic has full test coverage.
* All permalinks, paginator URLs etc. are then built on top of that same logic.

Fixes #1252
Fixes #2110
Closes #2374
Fixes #1885
Fixes #3102
Fixes #3179
Fixes #1641
Fixes #1989

output/outputType.go

@@ -27,6 +27,7 @@ var (
 		Name:      "AMP",
 		MediaType: media.HTMLType,
 		BaseName:  "index",
+		Path:      "amp",
 	}
 
 	CSSType = Type{
@@ -43,7 +44,7 @@ var (
 
 	JSONType = Type{
 		Name:        "JSON",
-		MediaType:   media.HTMLType,
+		MediaType:   media.JSONType,
 		BaseName:    "index",
 		IsPlainText: true,
 	}
@@ -52,6 +53,7 @@ var (
 		Name:      "RSS",
 		MediaType: media.RSSType,
 		BaseName:  "index",
+		NoUgly:    true,
 	}
 )
 
@@ -112,3 +114,7 @@ func GetTypes(keys ...string) (Types, error) {
 
 	return types, nil
 }
+
+func (t Type) BaseFilename() string {
+	return t.BaseName + "." + t.MediaType.Suffix
+}

output/outputType_test.go

@@ -30,6 +30,7 @@ func TestDefaultTypes(t *testing.T) {
 	require.Equal(t, media.RSSType, RSSType.MediaType)
 	require.Empty(t, RSSType.Path)
 	require.False(t, RSSType.IsPlainText)
+	require.True(t, RSSType.NoUgly)
 }
 
 func TestGetType(t *testing.T) {