[ticket/11362] Correctly sanitise the directory path

We need to correctly remove ../ form the path if possible by removing the
previous folder aswell. Otherwise the finder is unable to locate /adm/style
directories in extensions as he is looking for /adm/../adm/style instead.

PHPBB3-11362
[ticket/11362] Add unit test for ../ in directory paths

PHPBB3-11362
[ticket/11362] Move phpbb_clean_path into a simple filesystem service

PHPBB3-11362
[ticket/11362] Use new filesystem class in finder

PHPBB3-11362
[ticket/11362] Replace other calls to phpbb_clean_path

Need to instantiate the object manually here, as the container is not yet
set up.

PHPBB3-11362
[ticket/11362] Extension manager depends on filesystem

PHPBB3-11362
[ticket/11362] Fix service description of finder

PHPBB3-11362
[ticket/11362] Add compatibility function phpbb_clean_path() again

The function first depends on the container, but also works without it and
without autoload. The reason for this is, it might be used before that
stuff is set up, like it has been in our common.php

PHPBB3-11362
[ticket/11362] Use container when available instead of creating a new instance

PHPBB3-11362
Merge pull request #1290 from nickvergessen/ticket/11362

Correctly sanitise the directory path in finder