Make PublishModule not inherit from JavaModule

[lihaoyi]

Similar to what we did for AssemblyModule/RunModule/TestModule. PublishModule should be something you can mix into JavaModule, but can also be used separately if you want to publish artifacts to maven central that already exist and don't need to be compiled on the spot. Mill's mill-dist artifact is an example of this, being an uberjar that doesn't fit into the normal JavaModule framework

[lefou]

Does it need to be an CoursierModule at least?

[lihaoyi]

@lefou not sure TBH, but in theory it should be able to define the things it needs as abstract and have them wired up automatically when mixed in with a javamodule or coursiermodule