publish-unstable: Make publishing unstable also update

This eliminates the need for a separate, somewhat confusing job. Essentially,
this makes the publishing process idempotent: it will always either create the
publish if one doesn't exist, or it will update the existing ones.

This change also migrates a bunch of utility functions into the library, so we
can better standardize on how we handle things with aptly.

Change-Id: If6daadb1411c0af85dfbd6b82edbb0c1ed6c91b6