[`refurb`] Guard `hashlib` imports and mark `hashlib-digest-hex` fix as safe (`FURB181`) (#14694)
## Summary
- Check if `hashlib` and `crypt` imports have been seen for `FURB181`
and `S324`
- Mark the fix for `FURB181` as safe: I think it was accidentally marked
as unsafe in the first place. The rule does not support user-defined
classes as the "fix safety" section suggests.
- Removed `hashlib._Hash`, as it's not part of the `hashlib` module.
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## Test Plan
Updated the test snapshots