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)

Related issues

• Related to #8518 (closed)
• Is duplicate of #10748 (closed)

Original created by @intrigeri on 6297 (Redmine)