Up until now that has been `nix-shell -E 'with import <nixpkgs> {}; with import ./. {}; stdenv.mkDerivation { name="whatever"; buildInputs=[things i want]; }'`, and now it's just `nix run -f . things i want`.
There has always been `nix-shell -p things i want`, but that's for things in nixpkgs only, not things in your current directory.