Ref T10939. This isn't ideal because it's easy to confuse with zero ("O" vs "0") but I think this will mostly be read-only so it's probably one of the least-bad uses we could make of "O". We haven't really gotten into trouble with "I" (vs "1") for initiatives. Still, open to better ideas.
The goal here is to allow commit messages to include packages in some reasonable way, like Reviewers: O123 Package Name, epriestley, alincoln. The parser will ignore the "Package Name" part, that's just for humans. And I don't expect humans to type this, but when the use arc diff --edit or similar to update an existing revision, the reviewer needs to be represented somehow. It also needs to appear in the commit messages that arc land finalizes somehow.
I didn't hook up /O123 as a URI, but this should do everything else I think.