pkgs/top-level: expose current overlays in pkgs

[LnL7]

This enables inspection of the currently used overlays. Useful for
usecases where nixpkgs is imported multiple times.
eg. different channels

self: super:
let
  latest = import <nixpkgs-trunk> {
    inherit (super) config overlays;
  };
in
{
  hello-custom-latest = latest.hello-custom;
}

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[matthewbauer]

This is probably okay, although it makes me a little concerned that packages will start inspecting the overlays - which they definitely should not be doing.

[LnL7]

While technically possible, there's not much you can do with them directly since it's a list of functions. What I was referring to with inspection is for debugging in the repl or builtins.trace.

nix-repl> :p pkgs.overlays
[ «lambda @ /path/to/example-overlay.nix:1:1» ]