Define a task grouping ability
In Drupal's core support, migrations can have multiple tags that can be used to select a group of tasks (migrations). migrate_plus provided a "group" functionality (max one per "task") which is used by CLI/UI tools to group migrations for presentation, and also provides common default configuration to each member of the group. In Soong, I threw in a half-baked "subtask" functionality, with the idea being that a task containing subtasks could provide the shared configuration. On second thought, that seems unnecessarily complicated (it's causing me some trouble implementing the status command) - and, really, the grouping functionality and shared configuration functionality should be separate.
So, let's take out the subtasks and design our grouping functionality. Probably just a list of tags, and front-ends could by convention use the first tag listed for a task as the primary grouping on presentation.