snapshot: mark package sets entry with a leading "pkg_set:"

This is to make clear what they actually are. The repository ID
by itself is confusing
snapshot: also save the package set state in the autoproj log
ops/configuration: apply overrides information when importing package sets
git: always set branch, commit and tag information in #snapshot

This is to ensure that it properly overrides all version info set
in the package sets (and thus ensure that the commit is available)
snapshot: sort the version file

This ensures that we can easily compare two versions files from
different point in time.
use VCSDefinition#create_autobuild_importer instead of re-doing the work
snapshot: only add an entry to the autoproj log if something changed
fix autoproj versions --save