Ignore missing manifest entries in the built wheel cache (#654)
## Summary
This is more of a hypothetical problem, but the cache manifest could in
theory get out-of-sync with the contents on disk. This PR modifies the
`BuiltWheelMetadata` lookup to warn (but not fail) if the manifest
includes a wheel that no longer exists on disk. You can mimic this by
removing a wheel from the `built-wheels-v0` cache without modifying the
manifest correspondingly.
