Moved list of models inside AppConfig instances.

This commit is a refactoring with no change of functionality, according
to the following invariants:

- An app_label that was in app_configs and app_models stays in
  app_config and has its 'installed' attribute set to True.

- An app_label that was in app_models but not in app_configs is added to
  app_configs and has its 'installed' attribute set to True.

As a consequence, all the code that iterated on app_configs is modified
to check for the 'installed' attribute. Code that iterated on app_models
is rewritten in terms of app_configs.

Many tests that stored and restored the state of the app cache were
updated.

In the long term, we should reconsider the usefulness of allowing
importing models from non-installed applications. This doesn't sound
particularly useful, can be a trap in some circumstances, and causes
significant complexity in sensitive areas of Django.

tests/app_loading/tests.py

@@ -1,6 +1,5 @@
 from __future__ import unicode_literals
 
-import copy
 import os
 import sys
 from unittest import TestCase
@@ -17,14 +16,15 @@ class EggLoadingTest(TestCase):
         self.old_path = sys.path[:]
         self.egg_dir = '%s/eggs' % os.path.dirname(upath(__file__))
 
-        # This test adds dummy applications to the app cache. These
-        # need to be removed in order to prevent bad interactions
-        # with the flush operation in other tests.
-        self.old_app_models = copy.deepcopy(app_cache.app_models)
+        # The models need to be removed after the test in order to prevent bad
+        # interactions with the flush operation in other tests.
+        self._old_models = app_cache.app_configs['app_loading'].models.copy()
 
     def tearDown(self):
+        app_cache.app_configs['app_loading'].models = self._old_models
+        app_cache._get_models_cache = {}
+
         sys.path = self.old_path
-        app_cache.app_models = self.old_app_models
 
     def test_egg1(self):
         """Models module can be loaded from an app in an egg"""