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/authlib.php

@@ -1102,7 +1102,7 @@ function signup_setup_new_user($user) {
     $user->secret      = random_string(15);
     $user->auth        = $CFG->registerauth;
     // Initialize alternate name fields to empty strings.
-    $namefields = array_diff(\core\user_fields::get_name_fields(), useredit_get_required_name_fields());
+    $namefields = array_diff(\core_user\fields::get_name_fields(), useredit_get_required_name_fields());
     foreach ($namefields as $namefield) {
         $user->$namefield = '';
     }