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/adminlib.php

@@ -4311,7 +4311,7 @@ class admin_setting_users_with_capability extends admin_setting_configmultiselec
                     'This is unexpected, and a problem because there is no way to pass these ' .
                     'parameters to get_users_by_capability. See MDL-34657.');
         }
-        $userfieldsapi = \core\user_fields::for_name();
+        $userfieldsapi = \core_user\fields::for_name();
         $userfields = 'u.id, u.username, ' . $userfieldsapi->get_sql('u', false, '', '', false)->selects;
         $users = get_users_by_capability(context_system::instance(), $this->capability, $userfields, $sort);
         $this->choices = array(