Practically composable devShell modules

[srid]

I can't think of an easy to way to fully modularize the nix devShell, however I think we can get closer.

Unlike flake-parts mkFlake, we start with a base devShell (ie., whatever shellFor returns), and then build up the mkShellArgs values using the module system. So, instead of:

      devShell = {
        nativeBuildInputs = [
          config.mission-control.wrapper
          pkgs.pre-commit
        ];
        shellHook = ''
          FLAKE_ROOT="''$(${lib.getExe config.flake-root.package})"
          export FLAKE_ROOT

          (cd FLAKE_ROOT && ${config.pre-commit.installationScript})

          ${config.mission-control.banner}
        '';
      }

the user simply have to write:

      devShell = {
        module = {
          imports = [
            config.mission-control.devShellModule
            config.flake-root.devShellModule
            config.pre-commit.devShellModule
          ];
        };
      }

Those 3 modules (mission-control, flake-root, pre-commit) would export the associated config that will be merged. Like #82, we can't predict the order of merging, so things like shellHook could be problematic in some cases.

All of this can happen in separate repo, and haskell-flake can simply use that to evaluate the final mkShellArgs from the given module.

[srid]

Closing in favour of #135