after updating the repos it is often necessary to re-install packages
to take into account changes dependencies. This diff add support for doing
that, either as a standalone operation (bin/install) or together with repo
updating (bin/update --install).
I welcome feedback on whether, if this is accepted, we should update our doc at
https://docs.softwareheritage.org/devel/developer-setup.html to recommend using
bin/install instead of a manual pip command.