MDL-72163 admin: Manually include some interfaces

Where the autolodaer is not registered (ABORT_AFTER_CONFIG and
installation), some adminlib classes, traits, and interfaces need to be
manually loaded.

lib/adminlib.php

@@ -111,6 +111,11 @@ require_once($CFG->libdir.'/ddllib.php');
 require_once($CFG->libdir.'/xmlize.php');
 require_once($CFG->libdir.'/messagelib.php');
 
+// Add classes, traits, and interfaces which should be autoloaded.
+// The autoloader is configured late in setup.php, after ABORT_AFTER_CONFIG.
+// This is also required where the setup system is not included at all.
+require_once($CFG->dirroot.'/admin/classes/local/settings/linkable_settings_page.php');
+
 define('INSECURE_DATAROOT_WARNING', 1);
 define('INSECURE_DATAROOT_ERROR', 2);