Rewrote portions of the app- and model-cache initialisation to handle some corner cases. It is now possible to use m2m relations before everything is imported and still get the right results later when importing is complete. Also, get_apps() should always return the same results, so apps won't randomly disappear in the admin interface.

Also reorganised the structure of loading.py, since the number of global variables was exploding. The public API is still backwards compatible.

Fixed #1796 and #2438 (he claims, optimistically).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@5919 bcc190cf-cafb-0310-a4f2-bffc1f526a37