refactor(fun): Consistent use of depot.third_party vs. pkgs

In preparation for the solution of b/108, we need to consistently use
`depot.third_party` for packages that are only packed in the TVL depot
and `pkgs` for things that come from nixpkgs.

This commit cleans up a huge chunk of these uses in //fun

Change-Id: I45a7b392a9749fa7859ff5100dcea415bda807c3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2914
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

fun/paroxysm/docker/default.nix

@@ -1,6 +1,6 @@
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
-depot.third_party.dockerTools.buildLayeredImage {
+pkgs.dockerTools.buildLayeredImage {
   name = "paroxysm";
   contents = [ depot.fun.paroxysm ];
   config.Entrypoint = [ "${depot.fun.paroxysm}/bin/paroxysm" ];