resourceloader: Make ResourceLoader::makeLoaderRegisterScript() internal

* There is only a single non-test caller to this method in the entire
  codebase, and no callers elsewhere (Wikimedia Git, Codesearch).

  It's only used with an array, so remove the other unused code paths,
  and mark it internal (to my knowledge, nothing ever used it outside
  RL in the past, either).

* Add test coverage for the module indexing logic.

Change-Id: I9e0f95416d5b2fdd87323288231ee6d8c85d88e7