[ticket/12586] Use dot filter when trying to find available extensions

PHPBB3-12586
[ticket/12586] Limit depth to 2, we require vendor/extension/ext.php

PHPBB3-12586
Merge pull request #2491 from nickvergessen/ticket/12586

Ticket/12586 Filter files and depth when looking for extensions

* nickvergessen/ticket/12586:
  [ticket/12586] Limit depth to 2, we require vendor/extension/ext.php
  [ticket/12586] Use dot filter when trying to find available extensions