On install, update the .cabal file with the one from the index
(patch manually merged into the cabal-1.16 branch) 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.