Adds consolidated ``resolve_orm_path()`` util

This is done wrong in a few places, and this util makes it unmistakably
clear what the intent of the operation is.
Housekeeping: import order
Updates docstring
PEP-8
Implements use of new utilty
Removes docstring content that appears in README
Implements use of new utility
Housekeeping: spacing
Replaces use of magic constant
Deprecates old _get_field
Implements use of new utility
Use better idioms in code loop
Fixes select_related derivation