[_]debconf-show - custom apt/apt-get tab completion no longer working after upgrade

[Zenaan Harkness <zenaan@freedbms.net>]

Upgraded from Debian 8/Jessie to 9/Stretch.

Use aliases e.g. - these used to be apt-get, have updated to apt, but
in either case, the tab-completions no longer work:
alias agu='sudo apt update'
alias agi='sudo apt install'

Used to successfully use the following to make these and other
aliases tab-complete:
CUSTOM_APT_COMPLETIONS="agu agi ..."
for i in $CUSTOM_APT_COMPLETIONS; do
   #make-completion-wrapper debconf-show $i
   complete -F _debconf-show $i
   #complete -C debconf-show $i
done

It seems the function _debconf-show used to exist here:
. /etc/bash_completion


This no longer works, because the _debconf-show function is
apparently no longer available, and the command debconf-show doesn't
do the job (package name completion list creation) either.

Evidently _debconf-show has been replaced with something else.

Where can I find the apt/apt-get/etc completion wrappers, to know how
to update my aliases/alii ?

TIA,