JPMS compliance

[vietj]

Support JPMS modules.

This is a tentative and nice to have, maybe it is not possible due to use of class loader to load plugins or something similar.

• it seems possible to support this for the public API part of codegen (io.vertx.codegen.annotations + io.vertx.codegen.format).
• it does not seem trivial to make work a modular processor with the existing tooling
  • https://issues.apache.org/jira/browse/MCOMPILER-412
  • https://stackoverflow.com/questions/53084037/annotation-processor-discovery-process-since-java-9