It seems to have been lost in 88d47bbd69.
Probably by accident?
I also pulled the literal bits into the format string. It looks more
readable to me that way. I'm happy to remove that if it's not desired.
Previously, if we ran a command like `asdf install` and we encountered a
plugin that didn't exist, asdf produces an error saying "No such
plugin". Without knowing which plugin it could be referring too, we'd
have to manually go through each plugin in `.tool-versions` to find the
culprit.
With this commit, we'll now also include the plugin name as part of the
messaging for easier debugging.
* 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
