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.

django/contrib/admin/__init__.py

@@ -7,35 +7,8 @@ from django.contrib.admin.sites import AdminSite, site
 from django.contrib.admin.filters import (ListFilter, SimpleListFilter,
     FieldListFilter, BooleanFieldListFilter, RelatedFieldListFilter,
     ChoicesFieldListFilter, DateFieldListFilter, AllValuesFieldListFilter)
+from django.utils.module_loading import autodiscover_modules
 
 
 def autodiscover():
-    """
-    Auto-discover INSTALLED_APPS admin.py modules and fail silently when
-    not present. This forces an import on them to register any admin bits they
-    may want.
-    """
-
-    import copy
-    from importlib import import_module
-    from django.conf import settings
-    from django.utils.module_loading import module_has_submodule
-
-    for app in settings.INSTALLED_APPS:
-        mod = import_module(app)
-        # Attempt to import the app's admin module.
-        try:
-            before_import_registry = copy.copy(site._registry)
-            import_module('%s.admin' % app)
-        except:
-            # Reset the model registry to the state before the last import as
-            # this import will have to reoccur on the next request and this
-            # could raise NotRegistered and AlreadyRegistered exceptions
-            # (see #8245).
-            site._registry = before_import_registry
-
-            # Decide whether to bubble up this error. If the app just
-            # doesn't have an admin module, we can ignore the error
-            # attempting to import it, otherwise we want it to bubble up.
-            if module_has_submodule(mod, 'admin'):
-                raise
+    autodiscover_modules('admin', register_to=site)