Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / system / tests / modules / plugin_test / src / CustomDirectoryExample2.php

<?php

namespace Drupal\plugin_test;

/**
 * Provides a test plugin within a custom directory.
 *
 * @Plugin(
 *   id = "custom_example_2",
 *   custom = "Meghan"
 * )
 */
class CustomDirectoryExample2 {}