Remove hacks when hashing package ids

Remove hacks when hashing package ids

Right now there's a few hacks here and there to "correctly" hash package ids by
taking a package's root path into account instead of the path store in the
package id. The purpose of this was to solve issues where the same package
referenced from two locations ended up having two different hashes.

This hack leaked, however, into the implementation of fingerprints which in
turned ended up causing spurious rebuilds. Fix this problem once and for all by
just defining hashing on package ids the natural and expected way.