ENH: Cleanup backend for Offsets and Period

This PR is a work in progress

Fixes #5306 - Removing unused imports
Fixes #5082 - Added `inferred_freq_offset`
Fixes #5028 - fixed issues with `_offset_map`
Fixes #4878 - `freq` parameter for `Period` init supports full set of Offsets.
Fixes #5418 - raise `DateParseError` rather than incorrect `KeyError` for invalid frequencies