8 files declare their use of PluginDefinitionInterface

CKEditor5PluginDefinition.php in core/modules/ckeditor5/src/Plugin/CKEditor5PluginDefinition.php
DefaultFactory.php in core/lib/Drupal/Component/Plugin/Factory/DefaultFactory.php
DefaultFactoryTest.php in core/tests/Drupal/Tests/Component/Plugin/DefaultFactoryTest.php
DefaultPluginManager.php in core/lib/Drupal/Core/Plugin/DefaultPluginManager.php
EntityTypeInterface.php in core/lib/Drupal/Core/Entity/EntityTypeInterface.php
LayoutDefinition.php in core/lib/Drupal/Core/Layout/LayoutDefinition.php
PluginDependencyTrait.php in core/lib/Drupal/Core/Plugin/PluginDependencyTrait.php
PluginDependencyTraitTest.php in core/tests/Drupal/Tests/Core/Plugin/PluginDependencyTraitTest.php

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