ENH: PeriodIndex resampling redux

[max-sixty]

As discussed here: #12874, some ideas:

• Only return PeriodIndexs when resampling PeriodIndex (BUG: Fix some PeriodIndex resampling issues #16153)
• Don't use convention when doing simple upsampling & downsampling ('simple' meaning a one to many relationship between subperiods and superperiods)
• Define how to handle 'complicated' resampling (meaning a many to many relationship between subperiods & superperiods, such as Week->Month)

ref: #7744, #12871, #8707, #15944

EDIT: strikethrough above after completion