Unify all the various haskellSrc2nix calls into a generic API that's
more convenient than packageSourceOverrides.

Note: I am skeptical of this change. It does include a couple of rather complex functions, which could be too much. But after #52848, I started to feel like the variants were getting a little out of hand and needed unifying. In particular, this also adds fairly generic support for revisions.

/cc @shlevy @mightybyte @infinisil

Motivation for this change

This enables a pretty convenient alternative to packageSourceOverrides with a lot more power.

haskellPackages.extend(genericSourceOverrides {
  # Gets foo from Hackage, using your sha256 for the tarball.
  foo.version = "0.1.0.0";
  foo.sha256 = "...";

  # Gets bar from Hackage, using all-cabal-hashes for sha256 and the cabal file.
  bar.version = "0.1.0.0";

  # Gets foobar from Hackage, using all-cabal-hashes for the
  # sha256, but the specified revision for the cabal file.
  foobar.version = "0.1.0.0";
  foobar.revision = "2";
  foobar.revisionSha256 = "...";

  # Builds baz from a custom source.
  baz.src = fetchFromGitHub { ... };
})

I am open to better names. Not sure generic* really captures the purpose quite right.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.