REF: move Resolution to tslibs.dtypes
This is pretty much a clean move, in preparation for making FreqGroup an enum and trying to de-duplicate our 3+ enum-like classes
At some point in one of the previous PRs, you said that deduplication might not be possible after all (https://github.com/pandas-dev/pandas/pull/34587#issuecomment-639530365). Can you give a bit of context on what your idea / goal is on that now? > Can you give a bit of context on what your idea / goal is on that now?

The mapping between FreqGroup and Resolution is not going to be a bijection, but im now thinking that both FreqGroup.FR_DAY and FreqGroup.FR_BUS map to Resolution.RESO_DAY.  This will allow us to clean up semi-kludge [here](https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/period.py#L520).
