Make tags non-required for fetching wheel metadata

[charliermarsh]

Summary

This looks like a big change but it really isn't. Rather, I just split get_or_build_wheel into separate get_wheel and build_wheel methods internally, which made get_or_build_wheel_metadata capable of not relying on Tags, which in turn makes it easier for us to use the DistributionDatabase in various places without having it coupled to an interpreter or environment (something we already did for SourceDistributionBuilder).