Allow annotations to inherit across namespaces

Previously it was not possible for annotations to extend other annotations. This is now possible by passing an array of $annotation_namespaces to \Drupal\Component\Annotation\Plugin\Discovery\AnnotatedClassDiscovery. This is also available on \Drupal\Core\Plugin\DefaultPluginManager

Example:

  public function __construct($type, \Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler, $annotation = 'Drupal\Component\Annotation\PluginID') {
    $plugin_interface = 'Drupal\my_module\MyPluginInterface';
    $annotation_namespaces = ['Drupal\my_module\Plugin\Annotation', 'Drupal\my_module_two\Plugin\Annotation'];
    parent::__construct("Plugin/migrate/$type", $namespaces, $module_handler, $plugin_interface, $annotation, $annotation_namespaces);
  }