User getattr for getting reloader package.

The main __script__ may not have a package when running as setup
scripts

Closes #1600

src/werkzeug/_reloader.py

@@ -73,7 +73,7 @@ def _get_args_for_reloading():
     # Need to look at main module to determine how it was executed.
     __main__ = sys.modules["__main__"]
 
-    if __main__.__package__ is None:
+    if getattr(__main__, '__package__', None) is None:
         # Executed a file, like "python app.py".
         if os.name == "nt":
             # Windows entry points have ".exe" extension and should be