Rework mc-rtc-superbuild devshell

[arntanguy]

This PR is attempting to simplify how devShells for mc-rtc-superbuild environments are built. In particular, the goal is to support:

1. creating a devShell with:

• all inputs inputsFrom required to build the chosen controller/plugins/observers/robots/states/...
• a valid configuration of mc_rtc.yaml to load them

2. from the same configuration generate a devShell containing every dependency built by nix (e.g "release")

The goal of 1. is local development, and 2. can be used to simply run the project from the binary cache.

In the spirit of flakoboros, I am (slowly) converging towards a flake module with a configuration like (in a consumer's flake)

        imports = [
          inputs.mc-rtc-nix.flakeModule
          {
            mc-rtc-superbuild =
              { pkgs, ... }:
              {
                enable = true;
                pname = "panda-prosthesis-superbuild";
                traceRuntimeDependencies = true;

                # Pass a function that takes pkgs!
                robots = [
                  pkgs.mc-panda-lirmm
                  pkgs.mc-panda
                ];

                apps = [
                  pkgs.mc-franka
                  pkgs.mc-rtc-magnum
                ];

                config = "lib/mc_controller/etc/panda_prosthesis/mc_rtc.yaml";

                devel = {
                  controllers = [ pkgs.panda-prosthesis ];
                  plugins = [ pkgs.panda-prosthesis ];
                  robots = [ pkgs.panda-prosthesis ];
                  config = "lib64/mc_controller/etc/panda_prosthesis/mc_rtc.yaml";
                };
              };

            flakoboros = {
              overrideAttrs.mc-panda = {
                src = inputs.mc-panda;
              };
              overrideAttrs.mc-panda-lirmm = {
                src = inputs.mc-panda-lirmm;
              };

              overrideAttrs.panda-prosthesis =
                { drv-prev, pkgs-final, ... }:
                {
                  src = lib.cleanSource ./.;
                  nativeBuildInputs = drv-prev.nativeBuildInputs ++ [ pkgs-final.pkg-config ];
                };
            };
          }
        ];

where:

• attributes directly under mc-rtc-superbuild.<attrName> as used for both devel and release shells
• attributes under mc-rtc-superbuild.devel.<attrName> are used as inputFrom in the devel shell, and merged with attributes in mc-rtc-superbuild.<attrName> for the release shell.

As-is it works and generates ${pname} and ${pname}-release devShells, but the implementation could be improved.

TODO:

• Rework CI for public/private/ccache variants