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
$THIRDPARTYROOT is identical content wise to $SERVERROOT unless you count where on line 184 its added as an include path as ```OC::$THIRDPARTYROOT . '/3rdparty' . PATH_SEPARATOR .```