Distinguish between public (API) and internal parts of Debezium

We should relocate any packages not intended for direct access by users or connector implementors under some impl or internal package.

That way we'll have more freedom when changing internal parts, without breaking any users/integrators who shouldn't – an in Java 9 cannot – depend on those internals. Also the API surface to be learned and understood by users gets smaller, making the library easier to get started with.