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/download.hh

@@ -0,0 +1,22 @@
+#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);
+
+}