Removed module-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.

tests/migrations/test_operations.py

@@ -1,4 +1,5 @@
 import unittest
+from django.apps import app_cache
 from django.db import connection, models, migrations, router
 from django.db.models.fields import NOT_PROVIDED
 from django.db.transaction import atomic
@@ -203,8 +204,8 @@ class OperationTests(MigrationTestBase):
         self.assertColumnNotExists("test_adflmm_pony", "stables")
         # Make sure the M2M field actually works
         with atomic():
-            app_cache = new_state.render()
-            Pony = app_cache.get_model("test_adflmm", "Pony")
+            new_app_cache = new_state.render()
+            Pony = new_app_cache.get_model("test_adflmm", "Pony")
             p = Pony.objects.create(pink=False, weight=4.55)
             p.stables.create()
             self.assertEqual(p.stables.count(), 1)