This commit adds a check that any class in X-Pack that is a feature
aware custom also implements the appropriate mix-in interface in
X-Pack. These interfaces provide a default implementation of
FeatureAware#getRequiredFeature that returns that x-pack is the required
feature. By implementing this interface, this gives a consistent way for
X-Pack feature aware customs to return the appopriate required feature
and this check enforces that all such feature aware customs return the
appropriate required feature.
Jason Tedor