This assumes that we will be installing to a Void Linux system with 'bash-completion' package installed.
Notes:
- sourcing the xbps bash completion for forward proofing.
- calling out to `_xbps_all_packages` and `_xbps_installed_packages` directly, as `_xbps_complete` expects $1 to be one of the xbps-* commnads.
- none of the extra `-args` are implemented due to the hardcoded $1 problem mentioned above.