resourceloader: Remove closure around $CODE.defineLoader()

resourceloader: Remove closure around $CODE.defineLoader()

Follows-up b7b84d55d4e, which embedded the whole of the three 'mediawiki'
JS files inside of startup.js, but did so inside of the pre-existing
closure that was there.

This adds some overhead we don't get value of, so best to remove
it and embed it as a sibling of startup.js in the same top-level
scope.

In local testing (Chrome stable, CPU 1/6th) this reduced startup
run-time from 73-78ms to 63-65ms.

Also:
* Declare isCompatible() as a normal function.
  Disable the implicit-globals warning given this isn't a regular module,
  this file is  intentionally in the global scope.
* Unfold the private startUp() function to its call site.

Change-Id: Ida51ab20898c9e4ae6cbf7ad2968d88d824a1483