Attempting to tab-complete an asdf command would result in a deprecation warning being printed.
The --authoritative/-A and --unauthoritative/-u flags have been removed from the `complete` command.
Context: https://github.com/fish-shell/fish-shell/pull/3660
* Rename `asdf which` -> `asdf current`
* Output `set by $path` with current command
* Use dummy plugin in current_command test
* Hide "set by" message if derived from legacy file
