Provide custom pkg db to Setup.hs

[angerman]

We currently build the Setup.hs with the host compiler. This works, but it would be nice if we could selectively augment the package database that Setup.hs is built with. That is e.g. allow to specify a newer Cabal version to build the Setup.hs with.

If we could do this, we would not need to patch Cabal in GHC, but could just patch it outside and have Setup.hs built against that Cabal.

--
This is technically a nix-tools issue. But as we are collecting stuff here, I post it here.

[ElvishJerricco]

In fact, we should treat Setup.hs a lot more like a normal component than we currently do. Ideally, it'd be almost indistinguishable from a normal component with build-type: Simple

[angerman]

We do this now.