dd-list: Use apt interfaces to access Sources files

As described in <20160111171230.GA17291@debian.org>, directly accessing
the files under /var/lib/apt/lists is error prone.  APT provides
command-line interfaces to both determine the names of files and get
their contents, regardless of whether or how they're compressed.

Signed-off-by: James McCoy <jamessan@debian.org>