lib: can depend on pkgs (a la nixpkgs#pkgs/pkgs-lib)

[blaggacao]

A tiny step carved out of thought journey while working on #145.

Benefit: we can encourage/document to extend ./lib more usefully:

example:

{ nixos, pkgs, ... }:
# [...]
{
  # [...]
  # Custom Libaray Utilities
  mkPlutoApp = pkgs.callPackage ./mkPlutoApp { };
}

[nrdxp]

looks interesting, don't have enough time to test right now. Will checkout later and look around.

[nrdxp]

This looks fairly benign, and since it's mostly documentation, which is always great, I'll go ahead and merge it. Thank you!