Kotlin: Make an internal function visible for unit tests

In case the tests are in a different module than the production code (which is common), what's the best way to make internal functions visible for tests?

In Java, I would have the production code and the test in the same package and make the methods-to-be-tested package-private (plus, add a @VisibleForTest annotation if the only reason for having it package-private rather than private is the test). Unfortunately, Kotlin doesn't have the concept of package-private.