jar tool can not build the following legal modular multi-release jar file because the validator won't validate the new public class q.Bar found in the versions/9 directory without a class with the same name in the base directory.  The package q is a concealed package.

META-INF/
META-INF/MANIFEST.MF
module-info.class
META-INF/versions/9/module-info.class
p/
p/Foo.class
p/Main.class
META-INF/versions/9/
META-INF/versions/9/p/
META-INF/versions/9/p/Foo.class
META-INF/versions/9/q/
META-INF/versions/9/q/Bar.class