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/classes/task/send_failed_login_notifications_task.php

@@ -115,7 +115,7 @@ class send_failed_login_notifications_task extends scheduled_task {
 
         // Now, select all the login error logged records belonging to the ips and infos
         // since lastnotifyfailure, that we have stored in the cache_flags table.
-        $userfieldsapi = \core\user_fields::for_name();
+        $userfieldsapi = \core_user\fields::for_name();
         $namefields = $userfieldsapi->get_sql('u', false, '', '', false)->selects;
         $sql = "SELECT * FROM (
                         SELECT l.*, u.username, $namefields

lib/classes/task/send_new_user_passwords_task.php

@@ -47,7 +47,7 @@ class send_new_user_passwords_task extends scheduled_task {
         // Generate new password emails for users - ppl expect these generated asap.
         if ($DB->count_records('user_preferences', array('name' => 'create_password', 'value' => '1'))) {
             mtrace('Creating passwords for new users...');
-            $userfieldsapi = \core\user_fields::for_name();
+            $userfieldsapi = \core_user\fields::for_name();
             $usernamefields = $userfieldsapi->get_sql('u', false, '', '', false)->selects;
             $newusers = $DB->get_recordset_sql("SELECT u.id as id, u.email, u.auth, u.deleted,
                                                      u.suspended, u.emailstop, u.mnethostid, u.mailformat,