[refurb] Guard hashlib imports and mark hashlib-digest-hex fix as safe (FURB181)

[sbrugman]

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.

Test Plan

Updated the test snapshots

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[charliermarsh]

This makes sense, thanks.