SPACK-38: Allow specs to be indexed by virtual dependencies.

- The following now work differently:

      spec['mpi']
      spec['blas']

  This can return a spec for openmpi, mpich, mvapich, etc., EVEN if
  the spec is already concretized.  This means that in a package that
  `depends_on('mpi')`, you can do `spec['mpi']` to see what it was
  concretized to.  This should simplify MPI and BLAS packages.

      'mpi' in spec
      'blas' in spec

  Previously, if the spec had been concretized, these would be `False`
  because there was not a dependency in the DAG with either of these
  names. These will now be `True` even if the spec has been
  concretized.  So, e.g., this will print "YES"

      s = Spec('callpath ^mpich')
      if 'mpi' in spec:
          print "YES"

- Similarly, this will be True:

      Spec('mpich').satisfies('mpi')

- Because of the way virtual dependencies are currently implemented,
  the above required some fiddling around with `package.py` so that it
  would never call `Spec.__contains__` (and result in endless
  recursion).

- This should be fixed by allowing virutal dependnecies to have their
  own package class.
  - This would allow a quicker check for vdeps, without a call to
    `all_packages`.
  - For the time being, `package.py` shouldn't call `__contains__`

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