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_writer.py

@@ -5,7 +5,7 @@ from __future__ import unicode_literals
 import datetime
 import os
 
-from django.apps.cache import cache
+from django.apps import app_cache
 from django.core.validators import RegexValidator, EmailValidator
 from django.db import models, migrations
 from django.db.migrations.writer import MigrationWriter
@@ -124,7 +124,7 @@ class WriterTests(TestCase):
 
         with override_settings(INSTALLED_APPS=test_apps):
             for app in test_apps:
-                cache.load_app(app)
+                app_cache.load_app(app)
                 migration = migrations.Migration('0001_initial', app.split('.')[-1])
                 expected_path = os.path.join(base_dir, *(app.split('.') + ['migrations', '0001_initial.py']))
                 writer = MigrationWriter(migration)