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-plus-ssl.nix

@@ -1,5 +1,5 @@
 # Common Lisp bindings to OpenSSL
-{ depot, ... }:
+{ depot, pkgs, ... }:
 
 with depot.nix;
 
@@ -21,7 +21,7 @@ in buildLisp.library {
     (buildLisp.bundled "sb-posix")
   ];
 
-  native = [ depot.third_party.openssl ];
+  native = [ pkgs.openssl ];
 
   srcs = map (f: src + ("/src/" + f)) [
     "package.lisp"