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`.
Yes, we could write a SymbolWarning trait or something that provides the Warning case class, but we'll anyway have to define a warning method to get the message, so I'm not sure whether this is actually an improvement.

Names aren't actually used for now so we can decide we don't need them. I thought they might provide a nice access-point for enabling/disabling rules, but maybe full class names are better.