splice.nix: add convenience functions

  newScope = extra: lib.callPackageWith (splicedPackagesWithXorg // extra);

  # prefill 2 fields of the function for convenience

  makeScopeWithSplicing = lib.makeScopeWithSplicing splicePackages pkgs.newScope;

  # generate 'otherSplices' for 'makeScopeWithSplicing'

  generateSplicesForMkScope = attr:

    let

      split = X: lib.splitString "." "${X}.${attr}";

    in

    {

      # nulls should never be reached

      selfBuildBuild = lib.attrByPath (split "pkgsBuildBuild") null pkgs;

      selfBuildHost = lib.attrByPath (split "pkgsBuildHost") null pkgs;

      selfBuildTarget = lib.attrByPath (split "pkgsBuildTarget") null pkgs;

      selfHostHost = lib.attrByPath (split "pkgsHostHost") null pkgs;

      selfHostTarget = lib.attrByPath (split "pkgsHostTarget") null pkgs;

      selfTargetTarget = lib.attrByPath (split "pkgsTargetTarget") { } pkgs;

    };

  # Haskell package sets need this because they reimplement their own

  # `newScope`.

  __splicedPackages = splicedPackages // { recurseForDerivations = false; };