Closes: #779391

Closes: #778537

Closes: #779475

Closes: #779474

The `tool_required` decorator now raises an exception when
the command cannot be found. This enables more flexible handling.
Associated Debian package is now suggested in the comment.

The list of external tools is now available through the `--list-tools`
command-line option. We also use this output to generate the Recommends field.

Move the packages of those tools to Recommends section.

Comparing squashfs images could be interesting for
embedded Linux distributions like OpenWrt.

Added squashfs-tools only to Recommends, as it is
less relevant for Debian.

It only produces large diffs when file sizes
differ and has no real practical use.

This reverts commit d4dfb1f9.

Depends are not correctly generated anymore.

Otherwise, as Helmut pointed out, we will skip Haskell comments or other lines
during the diff.

Let's document possible exit codes while we are at it.

Closes: #774983