Create dd-lists of patched packages

* Add debian.sources.list for Debian experimental and unstable suites
* Create debian.apt directory to store Debian *_source_Sources files
* Create debian.apt.*.conf files pointing at the debian.apt directory
* Make the APT_CONFIG environment variable point at the appropriate
  debian.apt.*.conf file so that dd-list command can get data out of the
  debian.apt directory
* Retrieve debian_names of the packages from the sources.patches file of
  the specific derivative or from the aggregated sources.patches file in
  the var/ dir
* Pass the packages as arguments to the dd-list command
* Save the resulting dd-list and handle unknown packages if needed
* Document new files and directories in doc/README and etc/htaccess.*

As a result, the dd-lists contain the most up-to-date information about
the patched packages' maintainers and uploaders.

Fixes: commit a69fdb4a