toStack: freeze cabal hashes for Hackage

[kirelagin]

When generating stack extra-deps for dependencies that come from Hackage, use the ...@hash syntax to point to a specific revision of the cabal file which corresponds to the one stored in all-cabal-hashes from the pinned nixpkgs (i.e. the one used by callHackage when building with Nix).

This is needed for reproducibility: to make sure that stack and Nix use exactly the same version+revision of each Hackage package.