hackage-db: .cabal file extraction

[juhp]

Have you considered adding a method to extract the .cabal file of a package from the Hackage tarball? (I just finally discovered this package today via cabal-edit.)

I do this in cabal-file using hackage-security but its API is jolly involved. Maybe hackage-db could offer such functionality in an easier way?

[sternenseemann]

I think cabal2nix implements this in Distribution.Nixpkgs.Haskell.Hackage, but I'm a bit fuzzy on the details, as I've never touched that code myself.

I'd be happy to merge a PR that moves this code into hackage-db, if you think it's useful. This should be a bit easier now, given that both packages are in the same repository. Not sure when I would get to this myself.