These three rules in `concretize.lp` are overly complex:
```prolog
:- not provider(Package, Virtual),
provides_virtual(Package, Virtual),
virtual_node(Virtual).
```
```prolog
:- provides_virtual(Package, V1), provides_virtual(Package, V2), V1 != V2,
provider(Package, V1), not provider(Package, V2),
virtual_node(V1), virtual_node(V2).
```
```prolog
provider(Package, Virtual) :- root(Package), provides_virtual(Package, Virtual).
```
and they can be simplified to just:
```prolog
provider(Package, Virtual) :- node(Package), provides_virtual(Package, Virtual).
```
- [x] simplify virtual rules to just one implication
- [x] rename `provides_virtual` to `virtual_condition_holds`
de8e795563