Fix generation of module import path.

biscuit/core/util/apps.py

@@ -11,7 +11,7 @@ class AppConfig(django.apps.AppConfig):
 
         # Run model extension code
         try:
-            import_module('%s.model_extensions' % self.__class__.__module__.package.name)
+            import_module('.'.join(self.__class__.__module__.split('.')[:-1] + ['model_extensions']))
         except ImportError:
             # ImportErrors are non-fatal because model extensions are optional.
             pass