Add conflicting dummy packages osmocom-nightly and osmocom-latest, and
make all packages from each repository depend on the right one.
As usually, the latest packages will only get changed when a new release
appears. So the dependency will get introduced after tagging a new
release. I have tested in an own OBS namespace, that everything works as
expected.
Related: OS#2640
Change-Id: I79c45e798c10a65443b9fb9ecb54393d1918608a
