Allow package instance tags (e.g git_revision:abcdef….) to be used for package version resolution with a guarantee it doesn’t break in the future, even if this tag is (perhaps mistakenly) being applied to some other instance.

Design doc (sorry, googlers only): https://docs.google.com/document/d/1CmKrJtMmvSUN4hseEbR75GqcOoWshKzHJKGvs5aL-h4/edit