Fixed #1796 -- only load a single copy of each model, even when it is

referenced via different import paths. Solves a problem with many-to-many
relations being set up incorrectly. Thanks to Curtis Thompson, Luke Plant and
Simon Willison for some excellent debugging of the problem. Refs #2232 (may
also have fixed that).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@3202 bcc190cf-cafb-0310-a4f2-bffc1f526a37