Now that subtypes are registered via sub-packages, this TargetType is
not needed and should not be used. This removes it and updates the
target.Repository and targets.Service to use the updated code.
This was missed in eac433f7ff.
This moves all of the tcp target functionality into a tcp package. It
also establishes a Register function in the target package to allow
specific target types to be registered at initilazation time. When
registering, a target type needs to provide its Subtype, as well as some
RepositoryHooks that are used by the target Repository.
Timothy Messier