export-orig: Add download-orig-from-debian functionality

[Bryce Harrington]

export-orig sometimes can't find tarballs via pristine-tar or launchpad, giving a poor user experience as it requires them to manually find and download tarballs.

LP: #1935565 explores supporting use of uscan. As a user this is convenience since it's short, memorable, and can be run from the command line. However there are caveats to its use that make it ill suited as a general solution to this problem.

A more reliably correct approach is to manually download the tarball from Debian. There might be a more efficient mechanism, but what I typically do is browse on the internet to the debian package page for my package, find .dsc file for the version I want, and then dget it into a tmp directory. For example:

  https://tracker.debian.org/pkg/dovecot

  dget -x https://deb.debian.org/debian/pool/main/d/dovecot/dovecot_2.3.13+dfsg1-1.dsc

The reason I dget the .dsc rather than just wget the *.orig.tar.gz is for its extra dscverify work. It can also sometimes download additional pieces I didn't know I might need; the above example also downloads an orig tarball for pigeonhole, for instance.

This process is a bit cumbersome in practice, I suppose due to context switching between cli and web browsers. But I wonder if git ubuntu could do something analogous (or better) in situations where it wasn't able to find tarballs?