Make it possible to swap out plugin discovery mechanisms even with translations.

Problem/Motivation

At the moment you have different ways to define translatable strings in discovery mechanisms.

/**
 * Implements hook_foo_plugins().
 */
  return array(
    'label' => t('Label'),
  );

/**
 * @Plugin(
 *   label = @Translation("Label")
 * )
 */

so if you just want to swap out the discovery mechanism you can't because your code is either using $definition['label'] or just $definition['label']->get().

Proposed resolution

One way to fix it could be to add a __toString() method to the translation class, and require people to cast it, when it's needed,
thought i consider this as bad DX, not sure about the proper way.