ReproduciblePolicy rework for the migration to reproduce.debian.net

While tests.reproducible-builds.org has been doing a great job in bootstrapping the reproducibility problem in Debian, it does so by building twice in a row with varying build environment. What we are actually interested in is if the binaries that we ship can be reproduced. That's what reproduce.debian.net has started to do.

Because that's a whole new way to tackle the problem, the interface is different. Hence the need for this change.

To enable us to hint at the binary level, the logic need to be adapted for checking on binary level instead of at source level.

And let's add some test cases while we're at it.