Splice mapattrs optimize (#507232)

let

  inherit (lib.customisation) mapCrossIndex renameCrossIndexFrom;

  inherit (lib) mapAttrs;

  spliceReal =

    inputs:

        # The same pkgs sets one probably intends

        // inputs.buildHost

        // inputs.hostTarget;

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

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

      merge =

        name: defaultValue:

        let

          # `__functor__` for functions instead.

          defaultValue;

    in

    builtins.mapAttrs merge mash;

    mapAttrs merge mash;

  splicePackages =

    {