Issue #1987826 by jsacksick: Convert system_plugin_autocomplete() to a new style controller.

...s/system/lib/Drupal/system/Access/SystemPluginUiCheck.php

.../system/lib/Drupal/system/Controller/SystemController.php

core/modules/system/system.module

core/modules/system/system.routing.yml

core/modules/system/system.services.yml