REF: mix PeriodPseudoDtype into PeriodDtype
This will allow us to start getting rid of `get_freq_code` usages and related libfrequencies functions that are unnecessarily roundabout.
looks good. slightly concerned about the UNDEFINED case (e.g. we don't assert this anywhere)........ > slightly concerned about the UNDEFINED case

It also shows up in period.pyx as FR_UND (xref #34588), so i dont think we'll be able to get rid of it, but we should at least be able to get to a point with _fewer_ of these enum-like codes lying around