version comparator: make `@dev` satisfy `@x.y.z:`

Not sure if this is the case already, i would say it is reasonable to have this feature.
This would help to simplify

```
depends_on('<package>', when='@x.y.z:')
depends_on('<package>', when='@dev')
```

to

```
depends_on('<package>', when='@x.y.z:')
```

where `@x.y.z` is a version at which a dependency is introduced.
