Fixed #27142, #27110 -- Made makemigrations consistency checks respect database routers.

Partially reverted refs #27054 except for one of the tests as this
solution supersedes that one.

Thanks Shai Berger for the review.

tests/migrations/test_loader.py

@@ -4,12 +4,11 @@ from unittest import skipIf
 
 from django.db import connection, connections
 from django.db.migrations.exceptions import (
-    AmbiguityError, InconsistentMigrationHistory, MigrationSchemaMissing,
-    NodeNotFoundError,
+    AmbiguityError, InconsistentMigrationHistory, NodeNotFoundError,
 )
 from django.db.migrations.loader import MigrationLoader
 from django.db.migrations.recorder import MigrationRecorder
-from django.test import TestCase, mock, modify_settings, override_settings
+from django.test import TestCase, modify_settings, override_settings
 from django.utils import six
 
 
@@ -484,12 +483,3 @@ class LoaderTests(TestCase):
             ('app1', '4_auto'),
         }
         self.assertEqual(plan, expected_plan)
-
-    def test_readonly_database(self):
-        """
-        check_consistent_history() ignores read-only databases, possibly
-        without a django_migrations table.
-        """
-        with mock.patch.object(MigrationRecorder, 'ensure_schema', side_effect=MigrationSchemaMissing()):
-            loader = MigrationLoader(connection=None)
-            loader.check_consistent_history(connection)