Prepare for Packit automation
    
    - mark the Source section and the %setup line as being autogenerated.
    - change collection changelog handling. CHANGELOG.md will not be
      autogenerated from spec %changelog, it will be automatically
      synchronized by Packit from auto-maintenance when creating an update.
    - get rid of unneded Source tags by moving them into an include file.
      Its content will differ on RHEL and Fedora, on Fedora it is empty.
      This allows us to avoid keeping unneded sources in the repo, while
      complying with the rule that Source tags should not be conditional.