Make <nix/fetchurl.nix> a builtin builder

This ensures that 1) the derivation doesn't change when Nix changes;
2) the derivation closure doesn't contain Nix and its dependencies; 3)
we don't have to rely on ugly chroot hacks.

src/libstore/builtins.hh

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "derivations.hh"
+
+namespace nix {
+
+void builtinFetchurl(const BasicDerivation & drv);
+
+}