[ticket/12784] Allow the extensions to add a custom auto loader

PHPBB3-12784
[ticket/12784] Shearch the "vendor" folders and then the autoload.php files

PHPBB3-12784
Merge pull request #2674 from Nicofuma/ticket/12784

[ticket/12784] Allow the extensions to add a custom auto loader

* Nicofuma/ticket/12784:
  [ticket/12784] Shearch the "vendor" folders and then the autoload.php files
  [ticket/12784] Allow the extensions to add a custom auto loader