dillon1000/react

[rhir] move DeriveMinimalDependencies to its own file

--- 

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