ResourceLoader: expose registry as mediaWiki.loader.moduleRegistry

This patch exports the ResourceLoader registry object as a public member of
mw.loader. I understand that it is not properly a part of the public API
ResourceLoader exposes to JavaScript code; it is an internal data structure and
ResourceLoader does not expect to have anything modify it except ResourceLoader
itself. At the same time, it provides a trove of extremely valuable data for
debugging and profiling, and it's annoying as hell not to be able to poke and
prod it from the JS console. So my hope is that we do the Python thing of
treating 'private' as a tip and social convention rather than something that is
enforced by the code.

Change-Id: I1c47970aa9fceb0b2ded9008efc926b46fa08f51
(cherry picked from commit 9374fb34ef34d855e8d9c621547b2c09527b1950)