Fix some issues reported by lintian

See merge request ai3/tools/gitlab-deps!2

Changes-By: lintian-brush
Fixes: lintian: priority-extra-is-replaced-by-priority-optional
See-also: https://lintian.debian.org/tags/priority-extra-is-replaced-by-priority-optional.html

Changes-By: lintian-brush
Fixes: lintian: uses-debhelper-compat-file
See-also: https://lintian.debian.org/tags/uses-debhelper-compat-file.html

Changes-By: lintian-brush
Fixes: lintian: package-lacks-versioned-build-depends-on-debhelper
See-also: https://lintian.debian.org/tags/package-lacks-versioned-build-depends-on-debhelper.html
Fixes: lintian: package-needs-versioned-debhelper-build-depends
See-also: https://lintian.debian.org/tags/package-needs-versioned-debhelper-build-depends.html

Changes-By: lintian-brush
Fixes: lintian: missing-debian-source-format
See-also: https://lintian.debian.org/tags/missing-debian-source-format.html
Fixes: lintian: older-source-format
See-also: https://lintian.debian.org/tags/older-source-format.html

Remove a spurious match left over from previous commit.

Replaces an ugly AttributeError stack trace if you invoke the command
without arguments.

The server needs the reverse dependency graph, make sure the
update-gitlab-deps script generates it as well.

This isn't a JSON file.

For projects that do not have an associated repository.

Modular

See merge request ale/gitlab-docker-autodep!1

Improve help messages for the command too.

The idea is that all the filtering capability should be replaced by
"grep" and shell pipes.

Part one (server still TODO).