Add separate concern provider dependency for Wicket.

There is a crucial file io.csar.ConcernProvider which Csar uses to know which concern it should register. The GlobalMentor rincl-wicket repository provides an implementation for the Apache Wicket. As explained in RINCL-16, I had originally added the provider code into the actual rincl-wicket repository. But it became apparent that a better approach would be to split the io.csar.ConcernProvider file and its corresponding provider implementation into a separate repository.

Move io.csar.ConcernProvider (and the actual Java implementation it refers to inside) over to the new repository rincl-wicket-provider. Put the provider implementation into a provider subpackage. Copy over the POM and update the coordinates, the description, the repository URLs, and whatever else you need to. Don't forget to update the readme too.