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.

admin/tool/uploaduser/classes/process.php

@@ -155,7 +155,7 @@ class process {
             'interests',
         );
         // Include all name fields.
-        $this->standardfields = array_merge($this->standardfields, \core\user_fields::get_name_fields());
+        $this->standardfields = array_merge($this->standardfields, \core_user\fields::get_name_fields());
     }
 
     /**