Cabal files are now obtained from the index tarball (as cabal does)
directly. This allows us to always pick the cabal file revision that
cabal would pick, without having to understand or reference hackage.

…nd-more

Nix derivations are already able to pass a attribute to the builder as a
file. This means we don't need to use pkgs.writeText to turn the
cabalFile attribute into a file, saving one derivation.

…-and-more