SI-7019 Fix crasher with private[this] extension methods

When we move the body of value class methods to the corresponding
extension method, we typecheck a forward method that remains
in the class. In order to allow access, this commit weakens the
access of `private[local]` extension methods to `private`.
Merge pull request #4096 from retronym/ticket/7019

SI-7019 Fix crasher with private[this] extension methods