This adds a new requirement onto subtypes to register a NewFunc.
Previously the subtypes where implementing this function anyway, but the
target package was not including them in the registry. This resulted in
packages that use the target package needing to import the subtypes
directly, which we want to avoid.
This refactor will require the subtypes to register their NewFunc which
can then be used by `target.New` so packages that need to create targets
can do so by only using the target package.
This also updates the tcp subpackage to register a NewFunc.
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