Concrete to abstract classes w/ package private constructors

[dwijnand]

If/when MiMa understands Scala's notion of "package private" (good old #53) it would be good if the checks for concrete to abstract classes took into consideration that a concrete class with only a package private constructor can safely become abstract. This came up in the Vector rewrite of 2.13.2 (scala/scala#8534), specifically:

    // Vector was final, now sealed, and has no accessible constructor
    ProblemFilters.exclude[AbstractClassProblem]("scala.collection.immutable.Vector"),
...
    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.immutable.Vector.this"),
    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.immutable.Vector.apply"),

The missing (previously package private) constructor isn't a problem, as is the apply method only being abstract now instead of concrete.

However, this should also consider that abstract classes can extend concrete classes.