The Nix language itself is very small and simple; much of the functionality comes from the libraries which come bundled in the "nixpkgs" repository. The normal way to define a package is via the stdenv.mkDerivation function, which has accepted propagatedBuildInputs for at least the last few years (as long as I've been using Nix and NixOS).

To make things even easier, there are functions targeted at specific languages, like buildPythonPackage. That's taken propagatedBuildInputs for as long as I can remember, and since Python is interpreted that's basically the default way to use it (there are very few Python packages which are only required during build/installation time).

If the worst comes to the worst, you can always just define a "buildCommand" attribute containing arbitrary bash code. It'll be executed in a temporary directory, and inherit a bunch of environment variables (the most important being $out, which is the path which Nix will install).