7 files declare their use of DiscoveryInterface in Plug 7

AnnotatedClassDiscovery.php in lib/Drupal/Component/Annotation/Plugin/Discovery/AnnotatedClassDiscovery.php

Contains \Drupal\Component\Annotation\Plugin\Discovery\AnnotatedClassDiscovery.

DefaultFactory.php in lib/Drupal/Component/Plugin/Factory/DefaultFactory.php

Contains \Drupal\Component\Plugin\Factory\DefaultFactory.

HookDiscovery.php in lib/Drupal/Core/Plugin/Discovery/HookDiscovery.php

Contains \Drupal\Core\Plugin\Discovery\HookDiscovery.

InfoHookDecorator.php in lib/Drupal/Core/Plugin/Discovery/InfoHookDecorator.php

Contains \Drupal\Core\Plugin\Discovery\InfoHookDecorator.

PluginManagerInterface.php in lib/Drupal/Component/Plugin/PluginManagerInterface.php

Contains \Drupal\Component\Plugin\PluginManagerInterface.

YamlDiscovery.php in lib/Drupal/Core/Plugin/Discovery/YamlDiscovery.php

Contains \Drupal\Core\Plugin\Discovery\YamlDiscovery.

YamlDiscoveryDecorator.php in lib/Drupal/Core/Plugin/Discovery/YamlDiscoveryDecorator.php

Contains \Drupal\Core\Plugin\Discovery\YamlDiscoveryDecorator.