implement 'apt-get files' to access index targets

Downloading additional files is only half the job. We still need a way
to allow external tools to know where the files are they requested for
download given that we don't want them to choose their own location.

'apt-get files' is our answer to this showing by default in a deb822
format information about each IndexTarget with the potential to filter
the records based on lines and an option to change the output format.

The command serves also as an example on how to get to this information
via libapt.

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/apt-get.8.xml b/doc/apt-get.8.xml
index a372a0d30..da077afa7 100644
--- a/doc/apt-get.8.xml
+++ b/doc/apt-get.8.xml
@@ -243,6 +243,21 @@
        </listitem>
      </varlistentry>
 
+     <varlistentry><term><option>files</option></term>
+	<listitem><para>Displays by default a deb822 formatted listing of
+	      information about all data files <command>apt-get
+		 update</command> would download. Supports a
+	      <option>--format</option> option to modify the output format as
+	      well as accepts lines of the default output to filter the records
+	      by. The command is mainly used as an interface for external tools
+	      working with APT to get information as well as filenames for
+	      downloaded files so they can use them as well instead of
+	      downloading them again on their own. Detailed documentation is
+	      omitted here and can instead be found in the source tree in
+	      <literal><filename>doc/acquire-additional-files.txt</filename></literal>.
+	   </para>
+	</listitem>
+     </varlistentry>
 
 
    </variablelist>