resourceloader: Instantiate main class via ServiceWiring

resourceloader: Instantiate main class via ServiceWiring

It also removes some code duplication which is nice.

This unlocks various future changes, including:

* Making the `$config` parameter mandatory for the ResourceLoader class
  constructor, which currently falls back to global state.
  This should be deprecated and removed.

* Making it possible to instantiate the ResourceLoader class
  without all the default MW modules being registered from
  global state. E.g. move MW module registration from main class
  constructor to ServiceWiring, and remove the 'EmptyResourceLoader'
  class hack from unit tests, and use regular 'new ResourceLoader'
  instead.

* Making ResourceLoader a standalone library (some day),
  e.g. allowing it to be instantiated from a basic PHP script,
  in a way that is still useful and perhaps able to serve
  (most) RL modules without MW itself.

Bug: T32956
Change-Id: I4939f296c705b268e9cf8de635e923a739410470