We want to move towards having each functional domain in a separate crate.
The main benefit of that for our project is that this will enforce a unidirectional dependency graph (i.e. no cycles).
Starting off with virutal_branches - a lot of the implementation is still in core (i.e. virtual.rs), that will be moved in a separate PR.
Furthermore, the virtual branches controller (as well as virtual.rs) contain functions not directly related to branches (e.g. commit reordering etc). That will be furthe separate in a crate.
