xPDOTransport: Use new modFileHandler unpack() method

This is just an implementation of the method proposed here: https://github.com/modxcms/revolution/pull/11355

Tested with the package manager on a LAMP server with MODX 2.2.14pl, all 4 approaches of the new unpack() method (see PR above) worked for me.

@opengeek I didn't remove the "legacy" code as I'm not sure if an xpdo class should/is allowed to make use of modx classes...