splice: replace listToAttrs+map with mapAttrs (#506588)

        # The same pkgs sets one probably intends

        // inputs.buildHost

        // inputs.hostTarget;

      merge = name: {

        inherit name;

        value =

      # perf: mapAttrs defers merge calls until a key is selected, avoiding

      # ~60k eager closures that listToAttrs+map would create.

      merge =

        name: defaultValue:

        let

            defaultValue = mash.${name};

          # `or {}` is for the non-derivation attsert splicing case, where `{}` is the identity.

          value' = mapCrossIndex (x: x.${name} or { }) inputs;

          # Don't be fancy about non-derivations. But we could have used used

          # `__functor__` for functions instead.

          defaultValue;

      };

    in

    lib.listToAttrs (map merge (lib.attrNames mash));

    builtins.mapAttrs merge mash;

  splicePackages =

    {