This clarifies the intent and prepares for other functions doing the
same, like `next_display_order`. This will then open us to name
`subcommand_help_heading` and `display_order` similar.
The deprecation is waiting on 3.1.
This is part of #1807 and #1553.
This is prep for moving the derive tests. Besides organizing the test
folder for each API, this should reduce link time at the cost of
re-compiling more when a test changes.