Fixed bug #42: problem with satisfies() for virtual dependencies.
- _cross_provider_maps() had suffered some bit rot (map returned was ill-formed but still worked for cases with one vdep) - ProviderIndex.satisfies() was only checking whether the result map was non-empty. It should check whether all common vdeps are *in* the result map, as that indicates there is *some* way to satisfy *all* of them. We were checking whether there was some way to satisfy *any one* of them, which is wrong. - Above would cause a problem when there is more than one vdep provider. - Added test that covers this case. - Added `constrained()` method to Spec. Analogous to `normalized()`: `constrain():constrained() :: normalize():normalized()`