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.
Timothy Messier
c9b99306e1