hugolib, target: Rework/move the target package

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