Moved apps back in the toplevel django namespace.

Reverted 4a56a93cc4.

django/core/serializers/base.py

@@ -3,7 +3,7 @@ Module for abstract serializer/unserializer base classes.
 """
 import warnings
 
-from django.core.apps import app_cache
+from django.apps import app_cache
 from django.db import models
 from django.utils import six
 

django/core/serializers/python.py

@@ -5,8 +5,8 @@ other serializers.
 """
 from __future__ import unicode_literals
 
+from django.apps import app_cache
 from django.conf import settings
-from django.core.apps import app_cache
 from django.core.serializers import base
 from django.db import models, DEFAULT_DB_ALIAS
 from django.utils.encoding import smart_text, is_protected_type

django/core/serializers/xml_serializer.py

@@ -4,8 +4,8 @@ XML serializer.
 
 from __future__ import unicode_literals
 
+from django.apps import app_cache
 from django.conf import settings
-from django.core.apps import app_cache
 from django.core.serializers import base
 from django.db import models, DEFAULT_DB_ALIAS
 from django.utils.xmlutils import SimplerXMLGenerator