\Drupal\Core\Plugin\PluginDependencyTrait was added to \Drupal\search_api\Plugin\ConfigurablePluginBase in #2817341: ConfigurablePluginBase should use PluginDependencyTrait instead of DependencyTrait. The reason was that some plugins (that extend from that base class) wanted to use that trait (instead of just \Drupal\Core\Entity\DependencyTrait, which we used before that), but PHP 5 complained when doing that (as the same trait would then be added two different ways, something PHP 5 couldn’t handle).
Since we now only support PHP 7.0+, this safeguard is no longer necessary, and we can fall back to the trait we actually want to use for most plugins.
Any class inheriting from \Drupal\search_api\Plugin\ConfigurablePluginBase (directly or indirectly) that needs the extended trait’s methods (which are: getPluginDependencies, calculatePluginDependencies, moduleHandler and themeHandler) should just add \Drupal\Core\Plugin\PluginDependencyTrait to the class manually.
