extract-source: configure deb-src for the right dist

I'm working on a debian/-only (git-overlay) repository at:
https://salsa.debian.org/lts-team/packages/poppler/

Without upstream and pristine-tar branches, extract-source turns to origtargz, which uses various methods including apt-get source.

apt-get source only works for sid, as this is the dist that extract-source currently runs on (here failure for stretch):
https://salsa.debian.org/lts-team/packages/poppler/-/jobs/7730085

As discussed with @santiago adding deb-src matching the target dist could help.

A work-around is to rely on other methods (adding a pristine-tar branch at the cost of a heavy Git repo, relying on uscan --download-current-version).