Make Plexus dependencies 'provided' scope.

Plexus brings in JUnit 3.8.1 as a transitive dependency, which is overriding
Grails's 4.11 dependency in some circumstances. Since the plugin executes in
the context of Maven, Plexus is already available at runtime and so Plexus
doesn't need to be a runtime dependency. Hence why it's now provided scope.