gh-136547: fix `hashlib_helper` for blocking and requesting digests (#136762)

- Fix `hashlib_helper.block_algorithm` where the dummy functions were incorrectly defined.
- Rename `hashlib_helper.HashAPI` to `hashlib_helper.HashInfo` and add more helper methods.
- Simplify `hashlib_helper.requires_*()` functions.
- Rewrite some private helpers in `hashlib_helper`.
- Remove `find_{builtin,openssl}_hashdigest_constructor()` as they are no more needed and were
  not meant to be public in the first place.
- Fix some tests in `test_hashlib` when FIPS mode is on.