MDL-60671 core_search: Cron output should use mtrace

The mtrace function is preferable for plain text logging/progress
output because it can be redirected if necessary. By convention it
is normally used in cron and can be used in CLI tasks if required.

This change makes the text_progress_trace class use mtrace instead
of echo and then flush. (Default behaviour of mtrace is to do
exactly that, but it can be redirected too.)

Before this change, if anybody is relying on mtrace behaviour to
redirect logs of cron output, this will fail for some parts of the
search cron output (and some other places like auth plugins).

lib/weblib.php

@@ -3253,8 +3253,7 @@ class text_progress_trace extends progress_trace {
      * @return void Output is echo'd
      */
     public function output($message, $depth = 0) {
-        echo str_repeat('  ', $depth), $message, "\n";
-        flush();
+        mtrace(str_repeat('  ', $depth) . $message);
     }
 }