Determine custom vendor dir if vendor-dir is added/changed

Fixed #1347
This will point to the composer.json of phpDocumentor itself and not to the composer.json of the host application (because in that case phpDocumentor is installed in `/vendor/phpdocumentor/phpdocumentor`) and when you have installed phpDocumentor using PEAR then this file is located in the PEAR bin folder.

Can you move this determination to the Bootstrap class? That is always in the same place, regardless of installation method