- Accept (lists of) source packages as well as binary packages

- Accept (lists of) package names, and retrieve changelogs from
  get-changelog.cgi or similar (when it works).  In order for this to
  really work in apt mode, we need for apt to implement wishlist bug
  #80123.

- Use dpkg-parsechangelog once it's fixed to accept piped/gzip input
  (bug #80142).  This is not yet feasible because of bug #95579:
  dpkg-parsechangelog loses information.

- Multi-level sort: upload urgency, package priority, package name, what else?

- Make better use of python-apt
  control file parsing
  tar extraction

- What should be done in the situation where the installed version is
  newer than the seen version?  Currently, the seen version is used, but
  it might make more sense to use the installed version.
