Issue #2043379 by tim.plunkett: Allow plugins to be discovered in any directory.

core/lib/Drupal/Core/Action/ActionManager.php

core/lib/Drupal/Core/Archiver/ArchiverManager.php

core/lib/Drupal/Core/Condition/ConditionManager.php

core/lib/Drupal/Core/Entity/EntityManager.php

core/lib/Drupal/Core/Entity/Field/FieldTypePluginManager.php

core/lib/Drupal/Core/Menu/LocalActionManager.php

core/lib/Drupal/Core/Menu/LocalTaskManager.php

core/lib/Drupal/Core/Plugin/DefaultPluginManager.php

.../Drupal/Core/Plugin/Discovery/AnnotatedClassDiscovery.php

core/lib/Drupal/Core/TypedData/TypedDataManager.php

core/lib/Drupal/Core/Validation/ConstraintManager.php

.../lib/Drupal/aggregator/Plugin/AggregatorPluginManager.php

...dules/block/lib/Drupal/block/Plugin/Type/BlockManager.php

...es/ckeditor/lib/Drupal/ckeditor/CKEditorPluginManager.php

...ules/edit/lib/Drupal/edit/Plugin/InPlaceEditorManager.php

...modules/editor/lib/Drupal/editor/Plugin/EditorManager.php

...l/entity_reference/Plugin/Type/SelectionPluginManager.php

...al/field/Plugin/Type/Formatter/FormatterPluginManager.php

...b/Drupal/field/Plugin/Type/Widget/WidgetPluginManager.php

.../modules/filter/lib/Drupal/filter/FilterPluginManager.php