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

helpers/pathspec_test.go

@@ -52,7 +52,7 @@ func TestNewPathSpecFromConfig(t *testing.T) {
 	require.Equal(t, "no", p.language.Lang)
 	require.Equal(t, "side", p.paginatePath)
 
-	require.Equal(t, "http://base.com", p.baseURL)
+	require.Equal(t, "http://base.com", p.BaseURL.String())
 	require.Equal(t, "thethemes", p.themesDir)
 	require.Equal(t, "thelayouts", p.layoutDir)
 	require.Equal(t, "thework", p.workingDir)