hugolib, target: Rework/move the target package

This relates to #3123.

The interfaces and types in `target` made sense at some point, but now this package is too restricted to a hardcoded set of media types.

The overall current logic:

* Create a file path based on some `Translator` with some hardcoded logic handling uglyURLs, hardcoded html suffix etc.
* In in some cases (alias), a template is applied to create the alias file.
* Then the content is written to destination.

One could argue that it is the last bullet that is the actual core responsibility.

This commit fixes that by moving the `hugolib`-related logic where it belong, and simplify the code, i.e. remove the abstractions.

This code will most certainly evolve once we start on #3123, but now it is at least possible to understand where to start.

Fixes #3123

hugolib/hugo_sites_build.go

@@ -194,6 +194,8 @@ func (h *HugoSites) assemble(config *BuildCfg) error {
 func (h *HugoSites) render(config *BuildCfg) error {
 	if !config.SkipRender {
 		for _, s := range h.Sites {
+			s.initSiteWriter()
+
 			if err := s.render(); err != nil {
 				return err
 			}