(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 puppet 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 define RAKE_ROOT and use it since its used in more than
one place in the Rakefile.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>