Add a jpms module descriptor

[h908714124]

javapoet already declares an automatic module name com.squareup.javapoet.
However in order to be fully jpms-compliant, a module descriptor is needed.
Usually this is a file called src/main/java/module-info.java.

[JakeWharton]

It'll have to be in META-INF/versions/9/ since we support Java 8.