Orekit is still on Java 8 and so it can't be made modular (modularity i.e. Project Jigsaw came in Java 9) but to help projects that are modular use Orekit an entry should be added to the JAR manifest.
See http://branchandbound.net/blog/java/2017/12/automatic-module-name/. I just followed the advice under "Practical Tips" for maven and, after running mvn package, verified that the correct entry was added to the JAR manifest.