Adopt Java Platform Module System (JPMS)

The Java Platform Module System (JPMS; https://www.oracle.com/corporate/features/understanding-java-9-modules.html) has been introduced since Java 9. Cloud SDK for Java and Google API Client Libraries have not adopted the module system.

Example case: https://github.com/googleapis/google-cloud-java/issues/10975

# Workaround

Place the Cloud Java SDK in the class path, rather than the module path. The module path and the class path can coexist in a JVM.

# References

- https://github.com/grpc/grpc-java/issues/3522 When we evaluated few years ago, it got stuck with this gRPC issue about the module system. Now the issue has been resolved since last year.
- https://github.com/grpc/grpc-java/issues/2727#issuecomment-348209549 For gRPC, a workaround shared by a user was repackaging JAR files.