Make tags non-required for fetching wheel metadata (#2700)

## 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`).