Brought comments in sync with the code in BaseAppCache.
Moved django.db.models.loading to django.apps.cache.

This commit doesn't contain any code changes; it's purely a refactoring.
Removed BaseAppCache.app_store.

It was only storing redundant information. This is part of the effort to
allow applications without a models module.
Removed ModelDict.

Its only difference with OrderedDict is that it didn't deepcopy its
keys. However it wasn't used anywhere with models modules as keys, only
as values. So this commit doesn't result in any change in functionality.
Removed model-level functions for the app cache.

Since the original ones in django.db.models.loading were kept only for
backwards compatibility, there's no need to recreate them. However, many
internals of Django still relied on them.

They were also imported in django.db.models. They never appear in the
documentation, except a quick mention of get_models and get_app in the
1.2 release notes to document an edge case in GIS. I don't think that
makes them a public API.

This commit doesn't change the overall amount of global state but
clarifies that it's tied to the app_cache object instead of hiding it
behind half a dozen functions.