Follow-up to #17805
This replaces uses of pandas_datetime_to_datetimestruct and pandas_datetimestruct_to_datetime with dt64_to_dtstruct and dtstruct_to_dt64 respectively, but leaves some un-replaced to avoid creating conflicts with other open PRs.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
