Implement merging Packages files when extra suites are used.

anna's resolver (implemented in libdebian-installer) gets confused when
a package is available in several versions. Example: using wheezy and
wheezy-backports, debootstrap-udeb's configuration is attempted twice.

To mitigate this limitation, introduce merge_packages, which makes it
possible to merge both Packages files, preferring stanzas from the
overlay suite if there are any conflicts.

Of course taking the opposite path would be as easy, or even using a
mixture of packages (by specifying a list when building the image). For
now, assume udeb-producing packages are OK to pull from the overlay
suite, and possibly adding value compared to the base suite.

Note: A list of source packages could be set in src:debian-installer,
and used to generate a list of binary packages, to tweak the merging
algorithm. It's not entirely certain getting to this level of detail
is actually needed.

Signed-off-by: Cyril Brulebois <cyril@debamax.com>