Fix Engine.app_dir and leftover Backend.pdb()

Use __file__ instead of sys.argv[0] to determine the current script's
path, which works correctly when importing the package from another dir

harmonyqml/engine.py

@@ -26,7 +26,7 @@ class Engine(QQmlApplicationEngine):
         self._sigint_timer.start(100)
 
         super().__init__()
-        self.app_dir = Path(sys.argv[0]).resolve().parent
+        self.app_dir = Path(__file__).resolve().parent
 
         from .backend.backend import Backend
         self.backend = Backend(self)