feat(third_party): Add missing Quicklisp packages for Gemma

Gemma depends on cl-prevalence, which isn't in the nixpkgs Quicklisp
snapshot.

This adds the package and its dependencies to the overlay.

third_party/common_lisp/quicklisp.nix

@@ -0,0 +1,29 @@
+# Overlay over `pkgs.lispPackages` that adds additional packages which
+# are missing from the imported Quicklisp package set in nixpkgs.
+
+{ lib, lispPackages }:
+
+let inherit (lispPackages) buildLispPackage qlOverrides fetchurl;
+in lispPackages // lib.fix(self: {
+  "s-xml" = buildLispPackage
+    ((f: x: (x // (f x)))
+       (qlOverrides."s-xml" or (x: {}))
+       (import ./quicklisp-to-nix-output/s-xml.nix {
+         inherit fetchurl;
+       }));
+
+  "s-sysdeps" = buildLispPackage
+    ((f: x: (x // (f x)))
+       (qlOverrides."s-sysdeps" or (x: {}))
+       (import ./quicklisp-to-nix-output/s-sysdeps.nix {
+         inherit fetchurl;
+       }));
+
+  "cl-prevalence" = buildLispPackage
+    ((f: x: (x // (f x)))
+       (qlOverrides."cl-prevalence" or (x: {}))
+       (import ./quicklisp-to-nix-output/cl-prevalence.nix {
+         inherit fetchurl;
+         inherit (self) s-sysdeps s-xml;
+       }));
+})