This allows us to make minor changes to packages after they have been
released, without changing the package .tar.gz file. We already keep
the .cabal file outsite the package in the index and use it for
dependency planning. This already lets us do fixes such as making
dependency constraints tighter. Currently we cannot make dep
constraints more relaxed however, since the original .cabal file is
the one used when we get to the actual configure step.

So with this change, we now use the updated .cabal file for the
configure and build too. So there's more fixes we can do post-release.
In particlar, in combination with easier editing on hackage, this
should help us address the problems around the PVP and open or closed
version constraints. It should allow a system of conservative upper
bounds, but allow editing them when new versions of deps are released
and we find that they happen to work fine.