Add a flake check for hpack <-> `.cabal` consistency

[srid]

Based on #90 (comment)

Create a hpackCheck (analogous to the existing hlsCheck) if (and only if) the repository has both hpack and cabal files. The check will succeed only if running hpack generates the same cabal file.

This is useful in workflows where developers commit the generated cabal file to Git repo. The check will ensure that nobody has forgotten to commit package.yaml without the corresponding .cabal change.

[srid]

https://github.com/cachix/pre-commit-hooks.nix can also do this, indirectly.

[srid]

Unless someone raises a specific objection, I'll close this as wontfix because you can achieve the same functionality with pre-commit-hooks.nix, as we'll document it in #136