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/postmodern.nix

@@ -12,7 +12,7 @@ let
 
   cl-postgres = depot.nix.buildLisp.library {
     name = "cl-postgres";
-    deps = with pkgs.lisp; [
+    deps = with depot.third_party.lisp; [
       md5
       split-sequence
       ironclad
@@ -44,7 +44,7 @@ let
 
   s-sql = depot.nix.buildLisp.library {
     name = "s-sql";
-    deps = with pkgs.lisp; [
+    deps = with depot.third_party.lisp; [
       cl-postgres
       alexandria
     ];
@@ -58,7 +58,7 @@ let
   postmodern = depot.nix.buildLisp.library {
     name = "postmodern";
 
-    deps = with pkgs.lisp; [
+    deps = with depot.third_party.lisp; [
       alexandria
       cl-postgres
       s-sql