MDL-71099 Lib: Move new user_fields class from core to core_user

This class would belong more appropriately within the 'user' API
(core_user) instead of within the 'core' API, since it is
directly related to user data.

Since the class has only just been added to Moodle, now is a good
time to move it.

lib/accesslib.php

@@ -3923,7 +3923,7 @@ function get_role_users($roleid, context $context, $parent = false, $fields = ''
     global $DB;
 
     if (empty($fields)) {
-        $userfieldsapi = \core\user_fields::for_name();
+        $userfieldsapi = \core_user\fields::for_name();
         $allnames = $userfieldsapi->get_sql('u', false, '', '', false)->selects;
         $fields = 'u.id, u.confirmed, u.username, '. $allnames . ', ' .
                   'u.maildisplay, u.mailformat, u.maildigest, u.email, u.emailstop, u.city, '.