[Java] Support JPMS for Java 9+

[danepitkin]

Should Flatbuffers be modularized?

Flatbuffers is a low-level dependency for many Java projects. The Java Platform Module System (JPMS) was designed to have dependencies modularize bottom-up in the dependency graph. Currently, any Java project that implements JPMS and has flatbuffers-java as a dependency will depend on an auto-generated filename as the module name. This is problematic for any dependent Java project that is released publicly because it links the entire project stack to this implicit module name when the it should be explicit e.g. com.google.flatbuffers.

In the short term, #8348 can be implemented to explicitly set the module name before modularization is implemented.