Fatal error: Uncaught Error: Interface "Drupal\Core\DependencyInjection\ServiceProviderInterface" not found in /home/www/drupal8/test/modules/contrib/mailsystem/src/MailsystemServiceProvider.php:18 Stack trace: #0 {main} thrown in /home/www/drupal8/test/modules/contrib/mailsystem/src/MailsystemServiceProvider.php on line 18