False positive for mixin forwarders of package private trait methods

[lrytz]

Mixin forwarders for private[p] methods should not be reported

Test case in https://github.com/lightbend/mima/compare/main...lrytz:package-private-tests?expand=1

[dwijnand]

Reviewed this and we believe it to be a safe change, so MiMa shouldn't warn.