Issue #3303703: Deprecate the ComponentDiscovery class

src/ComponentDiscovery.php

@@ -43,6 +43,7 @@ class ComponentDiscovery {
    *   The application root directory.
    */
   public function __construct($app_root) {
+    @trigger_error(__CLASS__ . ' is deprecated in lightning_core:6.0.0 and removed in lightning_core:7.0.0. There is no replacement. See https://www.drupal.org/node/3303704', E_USER_DEPRECATED);
     $this->discovery = new ExtensionDiscovery($app_root);
   }