Fixed #21060 -- Refactored admin's autodiscover method to make it reusable.

We want to be able to use it for instance for discovering `tasks.py` modules
inside the INSTALLED_APPS.

This commit therefore moves the logic to `autodiscover_modules` method in
django.utils.module_loading.

tests/utils_tests/test_module/__init__.py

@@ -0,0 +1,3 @@
+class SiteMock(object):
+    _registry = {}
+site = SiteMock()