[1.7.x] Fixed #23483 -- Prevented ImproperlyConfigured with dotted app names

Made sure the app labels stay unique for the AppConfigStubs, so
migrations wouldn't fail if two dotted app names has the same last part
(e.g. django.contrib.auth and vendor.auth)

Backport of 5e32605ce9 from master.

django/db/migrations/state.py

@@ -124,6 +124,7 @@ class AppConfigStub(AppConfig):
     path = ''
 
     def __init__(self, label):
+        self.label = label
         super(AppConfigStub, self).__init__(label, None)
 
     def import_models(self, all_models):

tests/migrations/test_state.py

@@ -254,6 +254,27 @@ class StateTests(TestCase):
         with self.assertRaises(InvalidBasesError):
             project_state.render()
 
+    def test_render_unique_app_labels(self):
+        """
+        Tests that the ProjectState render method doesn't raise an
+        ImproperlyConfigured exception about unique labels if two dotted app
+        names have the same last part.
+        """
+        class A(models.Model):
+            class Meta:
+                app_label = "django.contrib.auth"
+
+        class B(models.Model):
+            class Meta:
+                app_label = "vendor.auth"
+
+        # Make a ProjectState and render it
+        project_state = ProjectState()
+        project_state.add_model_state(ModelState.from_model(A))
+        project_state.add_model_state(ModelState.from_model(B))
+        final_apps = project_state.render()
+        self.assertEqual(len(final_apps.get_models()), 2)
+
     def test_equality(self):
         """
         Tests that == and != are implemented correctly.