pep508: provide extra-only marker evaluation
We provide a new API on a `Requirement` that specifically
ignores the marker environment and only evaluates a requirement's
marker expression with respect to extras. Any marker expressions
that reference the marker environment automatically evaluate to
true.
Instead of duplicating the evaluation code, we just make a marker
environment optional on the lower level APIs. In theory, we could
just writer a separate evaluation routine that ignores everything
except extras, but the evaluator has a fair bit of other stuff in it
(such as emitting warnings) that would be good to keep DRY IMO.
