When resolving requirements, we now pass the list of extras we're using along
to Marker.evaluate, since we want to include the extra's requirements in our
list of required packages. This is sort of papering over the underlying issue;
namely, that the dependency map for dist-info distributions looks like:

  { None      : ['common_dep'],
    'my_extra': ['extra_dep; extra = "my_extra"'] }

If we eliminated 'extra = "my_extra"' when creating this map, the problem
would go away because the WorkingSet would no longer treat `extra_dep` as a
purely optional dependency. However, this would require copying and
manipulating Requirement objects, which is somewhat more complicated than the
current solution.