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/libexpr/download.hh

@@ -1,22 +0,0 @@
-#pragma once
-
-#include "types.hh"
-#include <string>
-
-namespace nix {
-
-struct DownloadResult
-{
-    bool cached;
-    string data, etag;
-};
-
-DownloadResult downloadFile(string url, string expectedETag = "");
-
-Path downloadFileCached(const string & url, bool unpack);
-
-MakeError(DownloadError, Error)
-
-bool isUri(const string & s);
-
-}