To bootstrap the Customizer, we've had to work on it as an app and inject our soon-to-be add-on code into this app. This is due to various limitations detailed in the depends-on bugs. With these dependencies resolved at least partially, we can switch the Customizer to being an add-on.
Created attachment 8579799 [details] Make the Customizer an add-on. I think we're ready for this. The dependencies will all be landing soon, and there's lots of problems with the Customizer as an add-on that we should get started on.
Comment on attachment 8579799 [details] Make the Customizer an add-on. LGTM.