- _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()`
7383bd393e