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()`