fixtures: fix non-working package-scope parametrization reordering

The `.parent` was incorrectly removed in
a21fb87a90, but it was actually correct
(well, it assumes that Module.path.parent == Package.path, which I'm not
sure is always correct, but that's a different matter). Restore it.

Fix #12328.

src/_pytest/fixtures.py

@@ -187,7 +187,8 @@ def get_parametrized_fixture_keys(
         if scope is Scope.Session:
             scoped_item_path = None
         elif scope is Scope.Package:
-            scoped_item_path = item.path
+            # Package key = module's directory.
+            scoped_item_path = item.path.parent
         elif scope is Scope.Module:
             scoped_item_path = item.path
         elif scope is Scope.Class: