Issue #1705702 by dawehner, damiankloip: Added Provide a way to allow modules...

Issue #1705702 by dawehner, damiankloip: Added Provide a way to allow modules alter plugin definitions.

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

@@ -51,7 +51,6 @@ public function getDefinitions() {
         $definitions[$plugin_id] = $definition;
       }
     }
-    drupal_alter($this->hook, $definitions);
     return $definitions;
   }
 }

core/modules/system/tests/modules/plugin_test/plugin_test.module

@@ -4,3 +4,13 @@
  * @file
  * Helper module for the plugin tests.
  */
+
+/**
+ * Implements hook_plugin_test_alter().
+ */
+function plugin_test_plugin_test_alter(&$definitions) {
+  foreach ($definitions as &$definition) {
+    $definition['altered'] = TRUE;
+  }
+  $definitions['user_login']['altered_single'] = TRUE;
+}