This clarifies what the 'create' method actually expects,
and adds a nicer approach for recipes that want to synthesize
their own package definition files.

BUG=

Change-Id: Iee5d5c02260bf71d4775ddfc1d4d1ae43937b693
Reviewed-on: https://chromium-review.googlesource.com/440385
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>