Do not call hpack if .cabal exists

[kirelagin]

I have a package where both package.yaml and .cabal are committed into the repository (as recommended nowadays). When I try to build it with haskell.nix, I get:

<pkgname>.cabal was generated with a newer version of hpack, please upgrade and try again.

From a quick look, it seems that there is some code that is meant to call hpack only if there is no .cabal file around, but, I guess, it does something else.

I have worked it around for now by doing:

{
  pkgname.cabal-generator = pkgs.lib.mkForce null;
}

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[hamishmack]

This has been fixed for cabalProject https://github.com/input-output-hk/haskell.nix/blob/9b73e5d8fbb7a79196b84d5db894547d977e8cab/lib/call-cabal-project-to-nix.nix#L527

I think someone needs to check if it has been fixed for stackProject.