Part of #1949932: [META] Unify entity fields and field API and followup to #1950632-42: Make formatters and widgets work on nonconfigurable fields:
In both cases (formatters / widgets), the rendering / form building flow for entities need to be reworked so that formatters / widgets are invoked on "base fields" as well. Right now, they are only invoked for "configurable fields", within field_attach_[prepare_view|view]() / field_attach[form|extract_form_values|form_validate]() respectively.
This issue is the start of that.