Restore wmf-packages MR, with additional stages fix

This is !5 (merged) again (which was briefly deployed then reverted), with an additional declaration of two unused stages (upload & release) in builddebs.yaml, otherwise pipelines fail to run with:

Pipeline cannot be run.
upload_ci_deb job: chosen stage does not exist; available stages are .pre, prepare, merge, build, test, deploy, .post

(even though they are when: never)