GitLab

* 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.

* Modifies the search logic for asdf to include Homebrew when it is
installed. The implementation is adapted from the pyenv plugin.