title: How can I get a list of repositories 'apt-get' is checking?
body: <p>I want a list of repositories in <em>sources.list</em>, plus those in <em>sources.list.d/</em>.</p>
  <p>Can I get this list in a form suitable for setting up
  another host so it watches the same repositories?</p>
  <p>Additionally, how do I determine which repository is the
  source of a package, either installed or available?</p>