Validate successful metadata fetch for direct dependencies (#5392)
## Summary
Prior to this change, the resolver would panic if we ran with
`--offline` and `--no-deps` and we had cached metadata for a _package_
(i.e., the versions) but no cached metadata for the _distribution_
(i.e., the specific wheel), since we weren't validating that the
returned metadata in the `--no-deps` case was actually successful. (We
need metadata, even for `--no-deps`, so that we can validate extras.)
## Test Plan
The added test panics on the previous branch.
