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
