(packaging) Use the packaging loader for tasks

The packaging repo now uses an explicit loader which handles loading the
various rake tasks in packaging. This commit updates the facter Rakefile to use
the loader instead of a blind glob of the ext/packaging/tasks directory. We
move this load into the rescue LoadError block because the packaging repo won't
always be there, and require 'rake' ahead of time.

This should not be merged until https://github.com/puppetlabs/packaging/pull/152 is merged.