Fatal error: Uncaught Error: Class "Drupal\migrate\Plugin\migrate\source\SourcePluginBase" not found in /home/www/drupal8/test/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php:56 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php on line 56