Fixed #17304 -- Allow single-path and configured-path namespace packages as apps.

Also document the conditions under which a namespace package may or may not be
a Django app, and raise a clearer error message in those cases where it may not
be.

Thanks Aymeric for review and consultation.

tests/apps/namespace_package_base/nsapp/apps.py

@@ -0,0 +1,8 @@
+import os
+
+from django.apps import AppConfig
+from django.utils._os import upath
+
+class NSAppConfig(AppConfig):
+    name = 'nsapp'
+    path = upath(os.path.dirname(__file__))