Check that packages are in the pool when ELA is published/changed

The CI pipeline should include a check that all versions of a package referenced in an ELA (whether it is the first publication or a later edit/change of the ELA) are present in the package pool.

This is presently documented as manual step for the developer wokflow:

+  Ensure the package was properly published to the `pool
+  <https://deb.freexian.com/extended-lts/pool/>`_ and available to
+  customers, e.g. with ``apt update && apt-cache policy yourpackage``.