Support relative paths to blueprint packages without publishing packages for those blueprints?

While developing a new series of blueprints, I haven't published anything yet, so... I don't think there is a way to test ember-cli-update for real (via actual invocation in the terminal).

My scenario:
- create default ember addon
- apply a number of tiny unpublished blueprint "diffs" that ember-cli-update can apply based on some CLI-based user interaction
- :tada: modified addon