Use GoalException not to print stack in case of external program failure
Move Goal.dependencies to a dedicated annotation (DependsOn)

This DependsOn annotation allow to change goal name / add new
dependencies without the need to redefine all supertype dependencies.
It also prevent from removing a supertype dependency.

Note that this depends on implementation of annotation inheritance mechanism.
Delete obsolete goalset for ceylon tasks
Introduce attachTo annotation.

This annotation can be used to attach a goal as a dependency of an
already defined goal.

Use case is when a workflow of goals is provided by a plugin and when
you want to attach a user-defined goal to a specific phase of this workflow.
Support of attachTo annotation in ceylon.build.runner