Fix a redundant warning with no keyrings
Checksum with dpkg-source instead of dscverify

This removes a lot of indirect dependencies.
Downside is that we could miss out on dpkg warnings.
Those will only be shown in case of outright failures
to avoid spamming the output with signature warnings.
Make installation instructions more portable

Ship a requirements file providing a valid version of
python-debian (if not already installed), and give
instructions for foreign distributions.
Add incremental import (git fetch)

Deepening shallow clones is still a work in progress.