Improve the tool import-debian-package to trigger the generation of debian/apertis/copyright on an initial import.

Background

When we import a package for a first time (not an update) in Apertis, the tool import-debian-package pushes directly to the apertis branch but it doesn't generate a copyright file, so the pipeline fails. This is why we need to manually create a MR to let the pipeline generate a copyright file. See pkg/libcloudproviders!1 (comment 145775)

We should change how import-debian-package works. It should create an apertis branch with only the commits importing upstream and debian versions (pipeline will fail but it's okay at this point), then it should create a branch proposed-blabla and the associated MR, thus the pipeline will generate a copyright file on the initial import. That is something nice to have before starting the rebase on Trixie.

Reproducibility

How often the issue is hit when repeating the steps to reproduce and changing nothing?

Put the ✅ in the most appropriate entry:

1. ✅ always
2. often, but not always
3. rarely

Impact of bug

More manual work when importing new packages.

Outcomes

TBD

Management data

This section is for management only, it should be the last one in the description.

/cc @andrunko @em @Balasubramanian @sudarshan @wlozano

Phabricator link: https://phabricator.apertis.org/T10687