When checking if a plugin is installed (to display a flag) it is not
necessary to loop through all installed plugins, since we already have
a name we can check directly.
Also expand test case to test this code path too.
When a shim is created, add plugin metadata so we can later know which shims belong to which plugins, this will help aid with removing unused shims on uninstall. See #67
```
# asdf-plugin: ${plugin_name}”
```
Thanks to @duijf for the metadata proposal.
* 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
