Move the OSBuildStagesToRPMs function, associated test, and RPM type
from the worker into the rpmmd subpackge. We will use this function in
the cloud API to compile the NEVRAs for the new metadata endpoint.
The Koji test in Github actions was always a bit quick and dirty solution.
I think it's much nicer solution to run it on Schutzbot.
Therefore, this commit moves the koji_test.go to a new osbuild-koji-tests
executable. This new test isn't run in the base test suite as one would
anticipate but inside the koji.sh test. This is needed because
osbuild-koji-tests requires a running koji instance. This might change
in the future but I think it works for now.
