Issue #3494360: Use ::class for class strings in plugin manager constructors.

Closes #3494360

Contains some optional commits that could be omitted, TBD.

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

@@ -33,7 +33,7 @@ class ActionManager extends DefaultPluginManager implements CategorizingPluginMa
    *   The module handler to invoke the alter hook with.
    */
   public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
-    parent::__construct('Plugin/Action', $namespaces, $module_handler, 'Drupal\Core\Action\ActionInterface', Action::class, 'Drupal\Core\Annotation\Action');
+    parent::__construct('Plugin/Action', $namespaces, $module_handler, ActionInterface::class, Action::class, 'Drupal\Core\Annotation\Action');
     $this->alterInfo('action_info');
     $this->setCacheBackend($cache_backend, 'action_info');
   }