Software design : Hiding the system from the user through packages

For Software Design class, we are required to design and implement a Bug Tracking system which undergoes several iterations.

We are taught to disconnect the back end (the bug tracking system) from the front end (UI) using a controller layer. We understood that if someone were to use our (back end) software and were to create a UI for it, (s)he should only be able to access the system via the controller layer. In other words, the classes in the software should be invisible.

Currently we are enforcing this by grouping our classes in packages and using the default/package access modifier.

I was wondering if this is the way to go or if there's a documented design pattern to achieve this encapsulation of front- and back-end? Are there any textbook examples on this?