Use templates for control files, release 10.0.0-3

Just a proof of concept for now.

My main question is whether the way I've hooked up the templating mechanism to the build process is acceptable. It feels pretty natural to me, but I've looked at other packages that do something similar (gcc, libreoffice) and they seem to generate the files as part of the clean target instead.

I'm pretty sure the CI pipeline will fail too, as it calls gbp buildpackage --git-no-hooks.

The other thing that I've noticed is that using dh-exec seems to make things significantly slower. On my machine, the build time goes from ~2m to ~2.5m, and that's with just one file using the facility. Hopefully the overhead doesn't grow linearly, but if that turns out to be the case it's probably going to be way too much.

Anyway, this is what I have at the moment. I'd love to hear your thoughts before investing more time on it.