Stop packagekitd from downloading updates

[Stefan Monnier <monnier@iro.umontreal.ca>]

Apparently, there's now a thing called `packagekit` whose daemon seems
to like to download updates "in the background" for me.

Thanks, but no, thanks.  This tends to occur at inopportune times for me
and it's not far enough "in the background", so it gets in the way
(furthermore, I like to download my packages with `debdelta` and
`packagekitd` doesn't know how to do that, AFAICT).

How can I stop those downloads?

Currently, I did

    systemctl mask packagekit

which might get the job done, but I don't really know what other impact
it might have, and I see that APT complains about it (tho it still
works fine, as far as I can tell):

    # LANG=C apt update
    Hit:1 http://deb.debian.org/debian stable InRelease
    Hit:2 http://security.debian.org stable-security InRelease
    Hit:3 http://security.debian.org testing-security InRelease
    Hit:4 http://deb.debian.org/debian testing InRelease
    Hit:5 http://deb.debian.org/debian unstable InRelease
    Error: GDBus.Error:org.freedesktop.systemd1.UnitMasked: Unit packagekit.service is masked.
    Reading package lists... Done
    Building dependency tree... Done
    Reading state information... Done
    All packages are up to date.
    #

Where can I say specifically that I don't want automatic background
download of updates?


        Stefan