1. upstream most of the changes (without the bits specific to debian and
  autopkgtest), since https://travis-ci.org/cyrusimap/cyrus-imapd has
  been failing for a while now: even though everything's green in
  Travis, the tests are not even *run* (see for instance
  https://travis-ci.org/cyrusimap/cyrus-imapd/builds/588427681).

2. convert Dockerfile.sid into a regular shell script, so we don't have
   to run inside docker. Over time this will significantly diverge from
   upstream's approach, and thus will require more maintenance.

   A good middle ground option would be to have a separate
   cyrus-no-docker test in d/test/control, alongside the existing
   cyrus-docker one; while cyrus-docker should be working at all times
   (because copying changes from upstream is easy), cyrus-no-docker
   could me marked "flaky".

3. replace as many cpan/built-from-source libraries and executables with
   their equivalent Debian packages.
