Create distinct graph nodes for each package extra (#3908)
## Summary
Today, we represent each package as a single node in the graph, and
combine all the extras. This is helpful for the `requirements.txt`-style
resolution, in which we want to show each a single line for each package
with the extras combined into a single array.
This PR modifies the representation to instead use a separate node for
each (package, extra) pair. We then reduce into the previous format when
printing in the `requirements.txt`-style format, so there shouldn't be
any user-facing changes here.
