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/rust-crates/default.nix

@@ -1,10 +1,10 @@
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
 let
   buildRustCrate = attrs@{
     edition ? "2018",
     ...
-  }: depot.third_party.buildRustCrate (attrs // {
+  }: pkgs.buildRustCrate (attrs // {
     inherit
       edition
       ;