CLN: rename _tz_localize_using_tzinfo_api, standardize patterns
Maximized places where code is exactly copy/pasted so its easy to check it stays in sync (since so far the prospect of de-duplicating the affected code is not looking good)

Small optimizations in _tz_convert_tzlocal_utc, which is renamed to reflect the fact that it is actually the General Case, not just tzlocal.
Thanks @jbrockmendel 