Warn if a plugin depends on a package that is already included in Cake #269
Description
only if the reference is using the wrong version, of course.
Something along the same lines has been proposed in nils-org/dependabot-cake-action#3.
One practical application would be cake-contrib/Cake.ExtendedNuGet#139 where, when the references to Cake.Core and Cake.Common are updated, the references to NuGet.Packaging and NuGet.Protocol must be bumped accordingly.