Have checkNotPrivate skip over private aliases.

[odersky]

CheckNotPrivate now avoids private aliases, so that types sat prepresentable in
transformation phases.

This does not solve the problem that private classes or abstract types might leak coming
from Scala 2, but there is not really a good cure for that. We can reject them outright or
allow them under language:Scala2 and hope for the best.

Fixes #1130

Review by @smarter

[smarter]

so that types sat prepresentable in transformation phases

I think there's a few typos in this description, what's "prepresentable"?

[smarter]

This does not solve the problem that private classes or abstract types might leak coming
from Scala 2, but there is not really a good cure for that.

So how come this works in Scala 2 with separate compilation? Is there something we should do differently in unpickling to see these private types?

[DarkDimius]

So how come this works in Scala 2 with separate compilation?

See related issues to https://issues.scala-lang.org/browse/SI-6794 I'm not sure it reliably works even without separate compilation.

[odersky]

So how come this works in Scala 2 with separate compilation?

I'll have to write this up in detail at some point. It's a fundamental issue.

[smarter]

LGTM.

I'll have to write this up in detail at some point. It's a fundamental issue.

Sounds interesting!