drupal/core/modules/migrate_drupal/migrate_drupal.module

<?php

/**
 * Implements hook_entity_type_alter().
 */
function migrate_drupal_entity_type_alter(array &$entity_types) {
  /** @var \Drupal\Core\Config\Entity\ConfigEntityType[] $entity_types */
  $entity_types['migration']
    ->setClass('Drupal\migrate_drupal\Entity\Migration')
    ->setControllerClass('storage', 'Drupal\migrate_drupal\MigrationStorage');
}