As an extension developer, I want Wikibase to use the same extension registration mechanism as every single other WMF-deployed extension.
As a MediaWiki developer, I don’t want to have to continue supporting non-JSON based extension registration.
As a WMF deployer, I want all registrations to be loaded the same way in production.
As a third-party Wikibase user, I want to load the Wikibase extension in the familiar, standard way.
The PHP entry point will still be there, but shouldn’t do much more than load the JSON file. We aim to make the PHP entry point optional – it should be possible to load the JSON file directly. (This may involve folding Lib into Client, see T254922 – that should be decided in an ADR [task to follow]. But we also want to check if it’s possible for Client’s JSON file to import Lib while it’s still a separate extension.)