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.

tests/template_tests/test_loaders.py

@@ -43,6 +43,8 @@ def create_egg(name, resources):
     """
     egg = types.ModuleType(name)
     egg.__loader__ = MockLoader()
+    egg.__path__ = ['/some/bogus/path/']
+    egg.__file__ = '/some/bogus/path/__init__.pyc'
     egg._resources = resources
     sys.modules[name] = egg
 
@@ -68,6 +70,9 @@ class EggLoaderTest(TestCase):
             def _get(self, path):
                 return self.module._resources[path].read()
 
+            def _fn(self, base, resource_name):
+                return resource_name
+
         pkg_resources._provider_factories[MockLoader] = MockProvider
 
         self.empty_egg = create_egg("egg_empty", {})