apt-setup is used to generate an /etc/apt/sources.list for the installed
system. It does this by creating a new sources.list file (commenting out
the previous contents) and then running each program in
/usr/lib/apt-setup/generators/ in turn (run-parts ordering). Each generator
is passed the name of a temporary file that it can write sources.list lines
(and comments) to.

After the generator finishes writing the file, apt-setup-verify will be run
on it to verify that each line of the sources works, and it will be
added to the sources.list. Generators can also run apt-setup-verify
themselves and do their own error recovery if it fails. apt-setup-verify
tests each line of the file and comments out lines that do not work, and
exits zero if all deb and deb-src lines in the file are ok, 30 if the
verification process was canceled, and otherwise nonzero on error.

Generators should add both "deb" lines and corresponding
deb-src lines to the sources.list. apt-setup-verify will handle commenting
out any deb-src lines for unavailable sources.

Generators can ask configuration questions using debconf. To support
backing up, generators should exit with the special return code of 10 if
the user backs up from their first question. Each generator should provide
a progress bar text template named apt-setup/progress/<generator>, where
<generator> is the script's name with leading numbers removed. Anything
after the first dot (if any) will also be removed when constructing the
progress template name, so that different scripts that share the same
purpose can easily be written.

Generators can advance the progress bar from PROGRESS_FROM up to
PROGRESS_TO, or it will be moved to the next step before the next
generator is started.

apt-setup-verify runs apt-get update inside debconf-apt-progress, to update
the progress bar with apt progress information. By default the progress bar
will not be moved. Use --from and --to options to override this. The values
of these options will be used as start and end points for the progress bar;
each source line to be tested will get an equal share. The resulting values
are passed on to debconf-apt-progress.
For example:
	apt-setup-verify --from 1 --to 100 <file>

Note that apt-setup and its generators may be run against some other system
in a chroot (i.e., when installing Debian). If apt-setup is running this
way, then ROOT will be set to the root of the chroot that it is acting on
and all sources.list validation will occur inside the chroot.

apt-setup is split into several udebs that contain different generators.
These udebs are installed depending on the type of install; for a install
from CD, cdrom-detect queues apt-cdrom-setup for install, for an install
from the network, choose-mirror queues apt-mirror-setup, etc.