[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.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

Updated the test snapshots

crates/ruff_python_semantic/src/model.rs

@@ -1411,6 +1411,8 @@ impl<'a> SemanticModel<'a> {
             "typing_extensions" => self.seen.insert(Modules::TYPING_EXTENSIONS),
             "attr" | "attrs" => self.seen.insert(Modules::ATTRS),
             "airflow" => self.seen.insert(Modules::AIRFLOW),
+            "hashlib" => self.seen.insert(Modules::HASHLIB),
+            "crypt" => self.seen.insert(Modules::CRYPT),
             _ => {}
         }
     }
@@ -2039,6 +2041,8 @@ bitflags! {
         const ATTRS = 1 << 25;
         const REGEX = 1 << 26;
         const AIRFLOW = 1 << 27;
+        const HASHLIB = 1 << 28;
+        const CRYPT = 1 << 29;
     }
 }