Welcome to the next level, naming stuff. Naming is one of the hardest things to do when developing and the Eclipse project has offered us the following guidelines.

Conventions from Eclipse Plug-in Developers Guide

The following package name segments are reserved:

• internal - indicates an internal implementation package that contains no API
• tests - indicates a non-API package that contains only test suites
• examples - indicates a non-API package that contains only examples

These name are used as qualifiers, and must only appear following the major package name. For example,

org.eclipse.core.internal.resources - Correct usage
org.eclipse.internal.core.resources - Incorrect. internal precedes major package name.
org.eclipse.core.resources.internal - Incorrect. internal does not immediately follow major package name.

They also have a convention of seperating out model from ui, so ui plugins get a .ui package name segment.

There is an example from udig:

• net.refractions.udig (model or core)
• net.refractions.udig.ui (user interface)

As a clue anything that is core can run in headless mode.

API rules of engagement
Naming Conventions(eclipse.org)