Complete Debian packaging transition to dh 12

[Daniel Kahn Gillmor]

This series follows the series introduced by
id:20191204084742.398298-1-dkg at fifthhorseman.net

Its goal is to move the notmuch debian packaging to dh 12.

To do this, the series accepts the conclusions about info files
reached in the thread anchored at id:87a7887akl.fsf at fifthhorseman.net,
and it also relies on dh_elpa having #946142 resolved, for example, by
merging https://salsa.debian.org/emacsen-team/dh-elpa/merge_requests/2
into that project.

What remains here is pretty simple mechanical work, which shouldn't
have an effect on anything else in notmuch other than how the debian
package itself gets assembled.

     --dkg