APIv4 - Deprecate nonstandard syntax for implicit joins

APIv4 - Deprecate nonstandard syntax for implicit joins

The APIv4 prototype used a string-manipulation hack to make implicit joins appear more friendly,
e.g. it would transform `contact_id.*` to `contact.*`.
However this made the generation of such joins less predictable,
as not all FK fields end in `_id` (notably, custom ContactRef fields do not).

This preserves the nonstandard syntax for backward-compat while favoring the new syntx
in the API Explorer.