chore(nix): Move files around to conform to new read-tree layout

Broadly speaking, the following things are included:

* there is now a uniform `args` struct that is passed to all
  derivations, package headers have been changed appropriately
* overrides are now loaded from a separate `override` folder just
  using read-tree.nix
* third-party packages have moved into the `third_party` attribute set

overrides/lispPackages/quicklisp.nix

@@ -0,0 +1,26 @@
+{ 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;
+       }));
+})