Add a dedicated method to retrieve the current version of the pkgdb API
Add method to retrieve the packages of a specific packager

This new method has been added in pkgdb API version 1.6 and allow to
more quickly retrieve the list of packages a packager is involved with.
Adjust the pkgdb-cli list --user to rely on the new API added in pkgdb 1.6

This makes retrieving the packages of a specified user much faster than
iterating over all his/her ACLs.
Fix the tests to account for the presence of the `acls` key even if it's empty