MDL-69166 core: define payment as a subsystem and pg as a plugin type

lib/classes/plugin_manager.php

@@ -1923,6 +1923,9 @@ class core_plugin_manager {
                 'quiz', 'resource', 'scorm', 'survey', 'url', 'wiki', 'workshop'
             ),
 
+            'pg' => [
+            ],
+
             'plagiarism' => array(
             ),