Fix for SPACK-13, and satisfies() now handles deps.

Added more test cases for multimethods.  In doing so, (re)discovered that
satisfies() really needs to handle dependencies properly.

Implemented support for dependencies in satisfies, but constrain() now
isn't consistent (as we do not currently constrain deps), so need to
implement that.

Virtual dependency support probably needs some deeper thought.  i.e.,
there is probably an intermediate DAG form that would make the needed
checks easier.  Right now we have to build ProviderIndexes to figure out
how virtual dependencies are set up.  If the vdep were preserved in the DAG,
then we could just check for things like incompatible providers directly.