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Small refactor of reactive dependency logic, no behavioral change.
- Moves ReactiveDependencyTree logic into `DeriveMinimalDependencies`.
- this moves hides most helper functions + types 🥳
- made `ReactiveDependencyTree` a class
- Changes `#dependencies` type: `Set<ReactiveScopeDep>` ->
`ReactiveDependencyTree`
- instead of collecting all dependencies into a tree in the end, we now eagerly
join dependencies into the tree on `visitDep`
- this is needed for the next PR in the stack, which relies on incremental
merging