Allow =version and /release selectors on virtual packages

The feature itself is not that big/important in my eyes (given the rambling the last few days on IRC some might disagree) and the code is kinda ugly… suggestions on how to improve that welcome!

It is nice to give it some consistency though which is why I tried to implement it as you could always do apt install debhelper-compat and apt would install debhelper, so it is certainly a bit strange that debhelper-compat=13 didn't work.