rebuild package on upstream package changes

[v4hn]

There is a long discussion (involving @jspricke 😁) on whether downstream packages should be/have to be rebuilt on package changes. As the OR build farm does it that way to account for potential breakage, it would be good to imitate that behavior.

As discussed with @rhaschke just now, this builder checks the package version available on the *-build repository and only builds a new package if that version is different from the one that would be generated in this run.
At that point in the process all potentially changed upstream packages are already built and available in the local action runner if they changed, so the only thing necessary would be to check whether any of the deb's in the local runner is a package dependency of the current package.

[rhaschke]

Done: a59cb69