Allow URLs in the Nix search path

E.g. to install "hello" from the latest Nixpkgs:

  $ nix-build '<nixpkgs>' -A hello -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz

Or to install a specific version of NixOS:

  $ nixos-rebuild switch -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/63def04891a0abc328b1b0b3a78ec02c58f48583.tar.gz

src/libexpr/download.hh

@@ -13,6 +13,8 @@ struct DownloadResult
 
 DownloadResult downloadFile(string url, string expectedETag = "");
 
+Path downloadFileCached(const string & url, bool unpack);
+
 MakeError(DownloadError, Error)
 
 }