turn None for the underlying InstallRequirement, as the base
   candidate will provide it, and we don't want to end up with duplicates.

The dependency on the base candidate is needed so that the resolver can't
decide that it should recommend foo[extra1] version 1.0 and foo[extra2]
version 2.0. Having those candidates depend on foo=1.0 and foo=2.0
respectively forces the resolver to recognise that this is a conflict.
N)