resourceloader: Apply safemode to startup module registry

resourceloader: Apply safemode to startup module registry

This effectively applies safemode to the mw.loader client,
without the client itself needing specific knowledge of safemode.

Test Plan:
* Unchanged: When viewing a page in safemode, the 'user' and
  'site' modules are still not queued by OutputPage.
* New: mw.loader.getState('site'), previously would yield
  'registered', but will now yield null.
* New: mw.loader.load('site'), previously loaded the module,
  it now logs a dependency warning for unknown module, like for
  any other unknown module.
* New: mw.loader.using('site'), previously resolved, it is now
  rejected.

Bug: T185303
Change-Id: I672e3891c8e1b3c2d13655fa134d0f1d031b8247