[packagekit] PiSi backend updated

[Richard Hughes]

Hey dude,

I've converted the PiSi backend to using a dispatcher to match the
others. It should "just work" if it's been kept up to date, but please
could you check it, and make any of the trivial changes needed in git
master. It should be much faster than before too. Yell if you need a
hand.

Thanks,

Richard.