top-level: Do not splice `pkgs`, `buildPackages` or `*Platform`

 - `pkgs` is self-similar, and thus already spliced
 - `buildPackages` is an ingredient of splicing and should be kept as is
 - The platforms are not packages or package sets and couldn't be spliced

There's probably other things that shouldn't be spliced too. The best long-
term solution is simply to stop splicing altogether.