Previously, the plugin exited early if the `asdf` command was not already
in $PATH:
(( ! $+commands[asdf] )) && return
This broke valid setups where `asdf` is installed manually in ~/.asdf
and not yet sourced globally (e.g., via .zshrc). Such installs are still
recommended in the official asdf documentation.
This change replaces the early exit with a check for the presence of
~/.asdf, so the plugin only runs if the user has a modern asdf setup,
while avoiding premature return before `$PATH` and completions are set up.
This preserves the new plugin behavior (removal of legacy <0.16 logic),
while restoring compatibility for the common manual install path.
* The check for the asdf installation directory is more precise:
The existence of the directory `$HOME/.asdf` does not mean that it is the installation
directory of `asdf`. It will also be created after installing at least one asdf plugin.
* Completions, while installed with homebrew, are now expected on an alternative location.