Fixed #21874 -- Require Django applications to have a filesystem path.

Wherever possible this filesystem path is derived automatically from the app
module's ``__path__`` and ``__file__`` attributes (this avoids any
backwards-compatibility problems).

AppConfig allows specifying an app's filesystem location explicitly, which
overrides all autodetection based on ``__path__`` and ``__file__``. This
permits Django to support any type of module as an app (namespace packages,
fake modules, modules loaded by other hypothetical non-filesystem module
loaders), as long as the app is configured with an explicit filesystem path.

Thanks Aymeric for review and discussion.

django/db/migrations/state.py

@@ -73,8 +73,11 @@ class ProjectState(object):
 
 class AppConfigStub(AppConfig):
     """
-    Stubs a Django AppConfig. Only provides a label and a dict of models.
+    Stubs a Django AppConfig. Only provides a label, and a dict of models.
     """
+    # Not used, but required by AppConfig.__init__
+    path = ''
+
     def __init__(self, label):
         super(AppConfigStub, self).__init__(label, None)