Rule for PackageObjectClasses

Warnings for the issue presented in SI-4344: https://issues.scala-lang.org/browse/SI-4344

Unlike the current `-Xlint` warning, this rule also warns about objects defined in package objects (as in the example at the link above). It is currently in `core` as was the case with `ByNameRightAssociative`.
Could the message be a parameter (maybe in a separate parameter list, but not necessarily)? I would like to avoid the boilerplate, if possible.

I agree with you on the name. I actually plan to implement `SuppressWarning` support these days, so a handle on the rule is needed. I tend to think the full name is probably better (since it solves the namespace issue).