Move fields functions out of the way
Notes:

- The only usage in `tslib` of `get_date_name_field` is for `Timestamp.weekday` and can be simplified with a more direct call.
- Several of the functions from `tslibs.fields` imported into `tslib` are unused.  Updating the external imports to use the new locations is for a follow-up.
- There is some flake8 cleanup to be done in the the new `tslibs.fields`, but that is being saved for a follow-up.  For now this is just cut/paste.
-  `get_start_end_field` will be largely unnecessary if/when the suggestion to deprecate Timestamp.freq in #15146 is adopted.

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