bpo-32691: Use mod_spec.parent when running modules with pdb (GH-5474)

Previously the module name was used, which broke relative imports when pdb was run against a plain module or submodule.

Lib/pdb.py

@@ -1532,7 +1532,7 @@ class Pdb(bdb.Bdb, cmd.Cmd):
         __main__.__dict__.update({
             "__name__": "__main__",
             "__file__": self.mainpyfile,
-            "__package__": module_name,
+            "__package__": mod_spec.parent,
             "__loader__": mod_spec.loader,
             "__spec__": mod_spec,
             "__builtins__": __builtins__,