Add method to retrieve the list of username of all packagers and information for a group

These results of these methods are cached for 1 hour and can be used to assess the
validity of a user/group to be point of contact for a package.
Add method to validate a point of contact

A valid point of contact is defined as:
- a user part of the packager group
- an existing group of type 'pkgdb'
Adjust unit-test (using mock) to still run withouth querying FAS to retrieve user/group info