Create a structure that has a) internal tasks (tasks that are essentially composable helper tasks) that do one specific task and are highly testable and b) has public tasks that are meant for end user consumption. At present this plugin simply uploads to AppCenter so that will be the only exposed public task.

Acceptance Criteria:

1. Tasks are grouped into an internal package
2. A single UploadTask exists in an outer package
3. Tasks are tested independently