We need virtuals because if we're going to have one class per compute resource, we'll end up declaring multiple copies of "foreman-compute" for the resources that don't have a specific package of their own (g.e if you included both Openstack and EC2)

I was going to copy the plugin declaration, but when using virtuals it just complicates the dependency chain - you end up ensureing all the Foreman::Compute defines are declared early *and* still have to do the Package realization.