http://c25o7knygjm3m67jy27yuynvv4pkfi25naucscmh4ubq2ggiig3v57ad.onion/en/supporting-java-modules-in-guix.html
It also allows for hiding internal classes and exporting only classes
that should be accessible outside the module. As I am not a Java developer, and I only care about building packages, the only
thing I need to know is that, whenever I see a module-info.java in the source
code, it requires Java 9 or later because it's using modules.