nix-env/nix-instantiate/nix-build: Support URIs

For instance, you can install Firefox from a specific Nixpkgs revision
like this:

  $ nix-env -f https://github.com/NixOS/nixpkgs/archive/63def04891a0abc328b1b0b3a78ec02c58f48583.tar.gz -iA firefox

Or build a package from the latest nixpkgs-unstable channel:

  $ nix-build https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz -A hello

src/libexpr/download.cc

@@ -223,4 +223,13 @@ Path downloadFileCached(const string & url, bool unpack)
 }
 
 
+bool isUri(const string & s)
+{
+    size_t pos = s.find("://");
+    if (pos == string::npos) return false;
+    string scheme(s, 0, pos);
+    return scheme == "http" || scheme == "https";
+}
+
+
 }