Improve 'requires /*runtime*/'

[jjohannes]

• For Javadoc: You currently have to switch to the runtime classpath for it to work
• For Whitebox tests (via java-module-testing): It's difficult to make it work as this reuses the module-info.class compiled by the main compile task.

If we keep the current solution with the doFirst action, we should make it configurable to which tasks the actions is added (so you can e.g. add it to the compileTestJava) and add it to the javadoc task by default as well.