Ensure that package structure is sound and that internal types are internal

[johnsonr]

We don't want unnecessary changes (and hopefully none affecting api package), but we do need to ensure that the package breakdown is as good as possible before we have to live with it forever with 1.0.

Considerations:

• Anything that is to be used by users must be under api
• Spring touchpoints should be in spring subpackages
• There are no packages with over 10 types
• Types not intended to be used by users should be internal
• Common types that are not agent specific go to embabel-common