Reimplement old XAR package plugin based on Xar and Filter modules

It was ok to keep two different implementations while testing the new one but now that we need to add new stuff to the XAR format it's going to be way too painful to maintain two completely different implementations. Best would be to refactor the old implementation to use the new Filter module behind the scene as much as possible and then implement XWIKI-12225 in the Filter module only.