Move internal components into their own namespace

[jbeemster]

It is meant to be exclusive - just for the core payload key-value pairs. The reason it gets confusing is that you can insert a TrackerPayload as the data component of a SelfDescribing json because it is really just a Map under the hood. We could probably clarify this by:

1. Renaming it to something more appropriate like NonEmptyStringMap
2. Removing the ability to pass this object directly into the SelfDescribingJson

Perhaps there is something around package visibility we should be toggling too...

cc/ @alexanderdean

[mscwilson]

This issue is broader than just TrackerPayload. I've changed the title to reflect that. We want to hide "private" parts of the code - in an internal namespace as suggested - to allow us greater flexibility in future for creating breaking changes. This change would also make the tracker easier to use with a better API.