No need to specify target types in the repo layout.

This is what I had in mind. I can build and run tests etc. with this change. But my understanding of why this was there in the first place is superficial, so it's possible I'm missing something.

E.g., this statement (from SetUpRepo.md) seems bogus:

"With this layout, if you build a `jvm_binary` target, it uses `src/java` as a base when figuring out whence to import `com.foo.amazing.service`; if you build a `java_library`, it uses `src/java` and `src/javatest` as bases."

What do you think?

BTW A pull request seemed like the easiest way to collaborate on a diff. No need to actually pull it...