Dmitry Kalinkin

I wonder if this can already be (partly) implemented as a wrapper applied in ```mkDerivation``` by using https://github.com/NixOS/nix/pull/1502 to define ```__toString```.

I see the opposite in my tests: ``` Welcome to Nix version 1.11.2. Type :? for help. nix-repl> let pkgs = import {}; in pkgs.fetchurl { url = "http://example.com/test.tar.gz"; sha256...