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

hugolib/hugo_sites_build.go

@@ -174,6 +174,12 @@ func (h *HugoSites) assemble(config *BuildCfg) error {
 	}
 
 	for _, s := range h.Sites {
+		for _, p := range s.Pages {
+			// May have been set in front matter
+			if len(p.outputTypes) == 0 {
+				p.outputTypes = s.defaultOutputDefinitions.ForKind(p.Kind)
+			}
+		}
 		s.assembleMenus()
 		s.refreshPageCaches()
 		s.setupSitePages()