(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 puppetdb 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.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #541 from MosesMendoza/maint/master/add_loader

(packaging) Use the packaging loader for tasks