reproducible: Add a PureOS default installation package set.

This currently uses the list of binary packages that are shipped with the
.iso image rather than inspecting the ISO itself (which might require
mounting, etc.) or what is actually installed on the system from the
"factory". However, I suspect this is highly accurate and is a good place
to start for now.

Signed-off-by: Holger Levsen <holger@layer-acht.org>