Autodetect "packages" based on cabal.project (and package.yaml)

[srid]

Resolves #76 & adds a projectRoot option.

This PR doesn't implement support for globs; users are expected to write down each package by hand in cabal.project. Latter we can improve this.

• Working Prototype
• Edge cases
  • Sometimes the cabal.project can live in a sub directory (eg: ./Backend). Do we support this case, and if so how? (EDIT: Can just add projectRoot option)
• Add tests for the parser; cf. https://www.tweag.io/blog/2022-09-01-unit-test-your-nix-code/
• Refactor, and maybe consider putting this code in external flake?
• Friendly error messages
  • On parse failure in particular instruct the user what to do rather than just throwing parse errors
  • If the package path has globs (*), print the limitation asking the user to set "packages" manually.
• Documentation

With this PR, haskell-flake gets its first flake dependency https://github.com/kanwren/nix-parsec which is slightly concerning, because the goal is to keep haskell-flake a thin abstraction on top of nixpkgs without growing to be a behemoth (eg: haskell.nix). (EDIT: Decided to just use fetchGit for this dependency, which is only evaluated if the user evaluates the default)

[srid]

Implementation works (no globs supported); what's left - refactoring, better error messages, and documentation update.

[peterbecich]

This is great, makes it easier to use haskell-flake, thanks!