Make repository.repositoryType public (repository.RepositoryType)

[eikenb]

I've been working with go-tuf a good amount recently and have wanted to use the repository.repositoryType abstraction, but cannot, as it is private (preventing passing it as an argument). I keep having to re-roll the functionality of this type locally and am wondering if there is a good reason why this is private... and if not, maybe change it to public.

[eikenb]

Guess I'll file this as a bug. It seems wrong to me.