* fix: don't invoke asdf inside asdf commands
Recursive calls have a number of disadvantages:
* Poorer performance since each invocation spawns and new process and re-executes all the code in bin/asdf
* Makes debugging more difficult
* More likely to introduce subtle bugs and the possibility for infinite loops
Issue: when asdf is first installed, there are no plugins, and running `asdf plugin-update --all` crashes because the plugin directory doesn't exist; similarly, the command crashes if the plugins directory exists yet doesn't contain any subdirectories.
Solution: This patch changes the directory loop from using a wildcard `plugins/*` to a more robust approach: first verify the plugins directory exists, then use the`find` command to list subdirectories and correctly handle the corner case of no subdirectories.
Future: Consider printing an error message and/or advice message. Consider automatically creating the plugins directory as needed. Consider prompting the user to install some popular plugins.
