Fixing 5150

Resolving bug where phpass failed to load if user has customized 3rdparty root via config file. Also updating constants paths to reflect correct paths/urls and making if logic on 135 uniform with rest of the system

Fixing https://github.com/owncloud/core/issues/5150
I'm aware that the logic is kind of weird - but  $THIRDPARTYROOT contains the folder where 3rdparty is living in