Hide some turbo_tasks internals (#5584)
### Description
This hides _some_ turbo_tasks internals from its public API.
It's currently not possible to completely hide them (hence
`Vc::into_raw`/unsafe `Vc::<AutoSet<RawVc>>::from_task_id`) because
turbo-tasks-memory still depends on some internal types, but this is a
step in the right direction.
