nixos/udev: use udev rules from $lib if present, otherwise $out

[flokli]

$bin is meant for executable binaries, not udev rules, and
./{lib,etc}/udev/rules.d also doesn't suggest for these to be in
$bin.

Motivation for this change

I was packaging something with multiple outputs and both $bin and $lib outputs, and was wondering myself a lot on why they weren't picked up.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[flokli]

I'd also be fine with this being $out all the time, but $bin clearly sounds wrong.

[flokli]

Also cc @ttuegel due to faf0d3e

[Valodim]

This seems like it might be a reasonable change. Some considerations:

• If we want to separate out udev rules, perhaps a udev output makes sense? I just checked and moolticute actually does this already, and seabreeze used to. The only reason to call this "lib" is FHS heritage, iinm, and a udev output feels more "typesafe", in a way.
• On the other hand, I suspect udev rules commonly trigger binaries, which will then be in their bin output, so we will anyways have that dependency in the majority of cases.

Either way, before merging this we need to make sure this doesn't break any packaged rules. Did you check that there is no derivation (besides bluez-qt) that packages its udev files into bin? I spotted libmtp as a candidate with nix-locate, at least.

[Valodim]

This documentation should definitely mention that this applies to files from the lib or default output.

[Valodim]

Suggested change

	now takes udev rules from <literal>lib</literal>, not <literal>bin</literal>.
	now takes udev rules from the <literal>lib</literal>, <literal>out</literal>, or default output (in that order), not <literal>bin</literal>.

[flokli]

Hm, a udev output might make sense, but do we then also want a lib.getUdev with fallback logic via lib, out and the default output?

Also, the same might apply for systemd.tmpfiles.packages and lib.getTmpfiles (once added) - maybe it's easier to put everything in $lib.

systemd.packages (and which outputs systemd units are picked from) also is somewhat inconsistent with what we plan here - any thoughts?

[flokli]

In a discussion on #nixos, it was proposed to fail building the nixos configuration if you try to assign a package (output) to services.udev.packages that doesn't expose anything in {etc,lib}/udev/rules.d (and handle tmpfiles and systemd.packages in a similar fashion). That way, moving around outputs to a more consistent layout would become less scary, as we don't accidentially silently miss and drop units or udev rules.

[flokli]

I'm still not sure which output to best put this into, and it certainly won't happen in 20.09 either. Let's close this for the time being.