Moved apps back in the toplevel django namespace.

Reverted 4a56a93cc4.

tests/swappable_models/tests.py

@@ -2,9 +2,9 @@ from __future__ import unicode_literals
 
 from django.utils.six import StringIO
 
+from django.apps import app_cache
 from django.contrib.auth.models import Permission
 from django.contrib.contenttypes.models import ContentType
-from django.core.apps import app_cache
 from django.core import management
 from django.test import TestCase
 from django.test.utils import override_settings