resourceloader: Add Doxygen group and improve overall docs

resourceloader: Add Doxygen group and improve overall docs

* Add license header where missing.
* Add missing `@since` (1.17 for most classes), except
  ResourceLoaderLessVarFileModule since 1.32 (1bc62c548c).

* Remove duplicate file-level description for class-only files,
  merge with the class description instead.

* Remove my own `@author` annotation from one file.

* Mark core's own FileModule subclasses as `@internal`, except
  for the following which we support use of in extensions:
  ResourceLoaderLessVarFileModule,
  ResourceLoaderOOUIIconPackModule, and
  ResourceLoaderWikiModule.

Change-Id: I336af2e4ccdbe2512594e8861b72628d24194e41