refactor(3p/lisp): Use sources from nixpkgs where possible

nixpkgs includes a lispPackages set which is generated from something.
In the meantime, we pretty much never update our Lisp deps.

This commit ties our sources to nixpkgs.lispPackages where the desired
package is included in nixpkgs (which is actually most of them!)

Change-Id: I520a006535980271b2fa4e0ed4e34029475dcbef
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4331
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

third_party/lisp/usocket-server.nix

@@ -0,0 +1,18 @@
+# Universal socket library for Common Lisp (server side)
+{ depot, pkgs, ... }:
+
+let
+  inherit (depot.nix) buildLisp;
+  src = with pkgs; srcOnly lispPackages.usocket-server;
+in buildLisp.library {
+  name = "usocket-server";
+
+  deps = with depot.third_party.lisp; [
+    usocket
+    bordeaux-threads
+  ];
+
+  srcs = [
+    "${src}/server.lisp"
+  ];
+}