Adding task group tasks to a variant's task list should be a validation error

To add a task group to a variant, you list that task group's name in the task list for the variant. This is resolved by the project parser into the constituent tasks, which are parsed as tasks in the task group. However, it is also possible to add the tasks individually to a variant. This is confusing because although it works, in the sense that the tasks will appear in the variant, these tasks will not be part of a task group, which is unexpected. We should make this a validation error.