[1.7.x] Fixed #22788 -- Ensured custom migration operations can be written.

This inspects the migration operation, and if it is not in the
django.db.migrations module, it adds the relevant imports to the
migration writer and uses the correct class name.

Backport of bb39037fcb from master

tests/migrations/test_writer.py

@@ -16,6 +16,9 @@ from django.utils.deconstruct import deconstructible
 from django.utils.translation import ugettext_lazy as _
 from django.utils.timezone import get_default_timezone
 
+import custom_migration_operations.operations
+import custom_migration_operations.more_operations
+
 
 class TestModel1(object):
     def upload_to(self):
@@ -222,3 +225,22 @@ class WriterTests(TestCase):
                 expected_path = os.path.join(base_dir, *(app.split('.') + ['migrations', '0001_initial.py']))
                 writer = MigrationWriter(migration)
                 self.assertEqual(writer.path, expected_path)
+
+    def test_custom_operation(self):
+        migration = type(str("Migration"), (migrations.Migration,), {
+            "operations": [
+                custom_migration_operations.operations.TestOperation(),
+                custom_migration_operations.operations.CreateModel(),
+                migrations.CreateModel("MyModel", (), {}, (models.Model,)),
+                custom_migration_operations.more_operations.TestOperation()
+            ],
+            "dependencies": []
+        })
+        writer = MigrationWriter(migration)
+        output = writer.as_string()
+        result = self.safe_exec(output)
+        self.assertIn("custom_migration_operations", result)
+        self.assertNotEqual(
+            result['custom_migration_operations'].operations.TestOperation,
+            result['custom_migration_operations'].more_operations.TestOperation
+        )