Fatal error: Uncaught Error: Class "Drupal\book\Plugin\migrate\source\Book" not found in /home/www/drupal8/test/core/modules/book/src/Plugin/migrate/source/d6/Book.php:21 Stack trace: #0 {main} thrown in /home/www/drupal8/test/core/modules/book/src/Plugin/migrate/source/d6/Book.php on line 21