Private types leaking in public signatures should be reported as errors even under -language:Scala2

[allanrenucci]

https://travis-ci.org/lampepfl/dotty-community-build/jobs/355301166

Regression introduced in #4013

[smarter]

The problem comes from a private type leaking into a public signature in scalatest, private leaks are errors on Dotty unless -language:Scala2 is set, then they're only warning. I'll try to patch scalatest to fix this. On the Dotty side, we probably need to make private leaks errors even under -language:Scala2.

[smarter]

Fix pushed: dotty-staging/scalatest@39c1f74

[liufengyun]

If we implement this check in Scala2 mode, then we can close #5831.