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/another_bad_module.py

@@ -0,0 +1,8 @@
+from . import site
+content = 'Another Bad Module'
+
+site._registry.update({
+    'foo': 'bar',
+})
+
+raise Exception('Some random exception.')