Sbuild::ResolverBase: Implement update_archive using apt-get update

David Kalnischkies pointed out in bug #714877 a clever way to make
apt-get update only affect a single sources.list-format file, using various
apt options to override the sources.list and sources.list.d locations,
and keep the lists for existing sources. This works "in any apt
version", and is cleaner than faking the list operations ourselves.
Implement --extra-package=package.deb

This simply adds a deb on the host system to the existing dummy archive
created for the build-dependency resolution metapackage, which makes it
available for resolving build dependencies. The purpose of this option
is to make it easier to build packages against locally-built build
depenencies, without waiting for those packages to enter the main
archive, or going through the hassle of maintaining a local archive and
making it accessible inside the chroot.

This option is named similarly to the EXTRAPACKAGES option in
pbuilderrc(5), but note that it takes a .deb, not a package name.

Thanks to Emanuele Aina for review!
Implement --extra-repository="deb scheme://..."

This simply adds another line to the sources.list.d file that lists our
dummy archive for build-dependency resolution, making that repository
available for providing build-dependencies. The purpose of this option
is to make it easier to build packages against a local archive without
modifying the source chroot to list that archive.
changelog for branch

Signed-off-by: Geoffrey Thomas <gthomas@mokafive.com>