create a skylark macro to ease java to scala dependencies

[johnynek]

See:

rules_scala/test/BUILD

Line 18 in c541451

jars = [":HelloLib_deploy.jar",

it is a pain to manually add all the deps back in. A skylark macro could construct that list:

scala_export_to_java(
  name = "my_scala_lib",
  exports = [":foo", ":bar"]
)

then we construct a java_library target as done here.

related to #3 #11