Load migration modules very python-like, handle replaces (ansible#670)

This fixes an issue where tests would error doing branch-switching incorrectly

test_app/tests/conftest.py

@@ -1,4 +1,3 @@
-import os
 import random
 import re
 from collections import defaultdict
@@ -54,15 +53,26 @@ def test_migrations_okay(*args, **kwargs):
             app_config = apps.get_app_config(app)
         except LookupError:
             raise RuntimeError(f'App {app} is present in the recorded migrations but not installed, perhaps you need --create-db?')
-        for path in os.listdir(os.path.join(app_config.path, 'migrations')):
-            if re.match(r'^\d{4}_.*.py$', path):
-                disk_steps[app].add(path.rsplit('.')[0])
+
+        migration_module = app_config.module.migrations
+        for step in dir(migration_module):
+            if not re.match(r'\d{4}_', step):
+                continue
+            step_module = getattr(migration_module, step)
+            migration_name = step_module.__name__.rsplit('.')[-1]
+            disk_steps[app].add(migration_name)
+
+            migration_cls = step_module.Migration
+            for replaces_app_name, replaces_migration_name in getattr(migration_cls, 'replaces', []):
+                disk_steps[app].add(replaces_migration_name)
+
     db_steps = defaultdict(set)
     for record in MigrationRecorder.Migration.objects.only('app', 'name'):
         if record.app in app_exceptions:
             continue
         app_name = app_exceptions.get(record.app, record.app)
         db_steps[app_name].add(record.name)
+
     for app in disk_steps:
         assert disk_steps[app] == db_steps[app], f'Migrations not expected for app {app}, perhaps you need --create-db?'