Save list of packages used at ISO build time
At ISO build time, generate a list of used packages and version, including packages used at build time but not shipped in the ISO.
See #5548 (closed) for implementation ideas and pointers to how others do.
Note that this highly depends on #9487 (closed): it might be that we decide it’s simpler to import the full APT suites we’re interested in, instead of a subset of their packages.
Feature Branch: feature/5926-freezable-APT-repository
Parent Task: #9489 (closed)