To allow Lucene to be modularised with Java module system there are a few preparatory tasks to be completed prior to this being possible. These are detailed by Uwe on the mailing list here:
The lucene-sandbox currently shares package names with lucene-core which is not allowed in the Java module system. There are two ways to deal with this. Either prefix all packages with "sandbox" or retire the lucene-sandbox all together. As per the email:
Cleanup sandbox to prefix all classes there with “sandbox” package and where needed remove package-private access. If it’s needed for internal access, WTF: Just move the stuff to core! We have a new version 9.0, so either retire/delete Sandbox stuff or make it part of Lucene core.
The suggested way forward is to move sandbox code to core.