Fixed #23384 -- Allowed overriding part of a dictionary-type setting

This change is needed for upcoming changes where settings might be
grouped in a parent dictionary.
Thanks Tim Graham for the review.

tests/migrations/test_executor.py

@@ -196,7 +196,7 @@ class ExecutorTests(MigrationTestBase):
     @override_settings(
         MIGRATION_MODULES={
             "migrations": "migrations.test_migrations_custom_user",
-            "django.contrib.auth": "django.contrib.auth.migrations",
+            "auth": "django.contrib.auth.migrations",
         },
         AUTH_USER_MODEL="migrations.Author",
     )

tests/migrations/test_loader.py

@@ -81,7 +81,10 @@ class LoaderTests(TestCase):
         # Ensure we've included unmigrated apps in there too
         self.assertIn("basic", project_state.real_apps)
 
-    @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_unmigdep"})
+    @override_settings(MIGRATION_MODULES={
+        "_clear_defaults": True,
+        "migrations": "migrations.test_migrations_unmigdep"
+    })
     def test_load_unmigrated_dependency(self):
         """
         Makes sure the loader can load migrations with a dependency on an unmigrated app.