PoC: Provide inputs as Nickel records

[YorikSar]

Generate Nickel record with all packages from each input and allow Nickel code to access them directly without declaring them in input_spec.

[YorikSar]

I've finally got this to somewhat work. Current limitations:

• have to limit nixpkgs input to only supported pkgs because of Poor performance on importing big records tweag/nickel#1428 and hack Haskell subset with recurseForDerivations fields. It seems we'd either have to evaluate everything in nixpkgs or somehow pick paths that are the most interesting for us.
• inputs are mixed into inputs field, it should be quite straightforward to add them to the lockfile, but then we'll need to pass it into builders.ncl somehow.
• files should be generated in separate derivations so that we can cache them and not re-evaluate then every time. I've tried just running nix-instantiate --eval --offline --extra-experimental-features flakes --store ./store "''${nixSourcePath}" > $out with nixSource set to:

        let
          nickel-nix = (builtins.getFlake "${./.}");
          input = builtins.getFlake "${input}";
        in
          nickel-nix.lib.${system}.collectPkgs input

  but that yields sandbox related errors. Probably it won't be a huge deal if we generate these files when generating the lockfile.

[YorikSar]

With tweag/nickel#1428 fixed with reported 10x speedup, we could get back to evaluating this approach. It might be nicer to provide such records to users with the lockfile so that completion works fine with them. We could then also expand them with additional functionality like wrapping passthru functions where needed. What do you think @thufschmitt?

[thufschmitt]

I would wait for 0.1 to be out as there's already a bit of work do be done first (besides the fact that we can't use the latest Nickel because of tweag/nickel#1664). But that would be great if it allows us to improve the Nix interop!

[yannham]

@YorikSar @thufschmitt what is the status of this PR? Is this work still relevant, but needs to be rebased, or can we just close it ?