Use java dir instead of kotlin dir for initial project generation

[chippmann]

Resolves #578

[CedNaru]

Why do we have to create our Kotlin scripts in a Java directory now ? So far, it has been working perfectly fine to have each language in its own directory. This feels like a terrible and nonsensical change.
Isn't that just up to the users to create their java files in the correct directory ? We don't have any responsability regarding that.
If we want to improve the IDE plugin to help people making less mistake, I'd rather just add an option in the setup wizard to generate a template script, not only for Kotlin, but also for Java. And they would be both directly created in their respective directory.