refactor(third_party): 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 //third_party

Change-Id: Ic382c0cdea7330a84d5f0b7d109c824ddceb94e7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2912
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/lisp/cl-prevalence.nix

@@ -1,8 +1,8 @@
 # cl-prevalence is an implementation of object prevalence for CL (i.e.
 # an in-memory database)
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
-let src = depot.third_party.fetchFromGitHub {
+let src = pkgs.fetchFromGitHub {
   owner = "40ants";
   repo = "cl-prevalence";
   rev = "da3ed6c4594b1c2fca90c178c1993973c4bf16c9";