Parse annotations recursively

Problem/Motivation

In the following annotation, the @Translation annotation is used both on the top level for 'label', but also nested in 'view_modes' -> 'full' -> 'label'.
Currently this does not work, preventing any nested values from being transltations.

use Drupal\Core\Annotation\Plugin;
use Drupal\Core\Annotation\Translation;

/**
 * Defines the user entity class.
 *
 * @Plugin(
 *   id = "user",
 *   label = @Translation("User"),
 *   module = "user",
 *   view_modes = {
 *     "full" = {
 *       "label" = @Translation("User account"),
 *       "custom_settings" = FALSE
 *     }
 *   }
 * )
 */
class User extends Entity {
  //...
}

Proposed resolution

Parse the annotations recursively, by splitting the actual parsing code out of Drupal\Core\Annotation\Plugin::__construct() and into a dedicated method.

Remaining tasks

n/a

User interface changes

n/a

API changes

Adds a new protected method Drupal\Core\Annotation\Plugin::parse().