Update rules to create empty python3 package

python3-ligo-common is no longer required, make it an empty package

@adam-mercer @duncanmmacleod as discussed via email, this ("option b.") is the best way to avoid surprises should someone come up with a new dependency on python3-ligo-segments

@steffen.grunewald, I will merge this for the record, but will then mark this project as archived and read-only, so that people know that it's dead.

mentioned in commit 33a08ae3

merged