Move rewritten appJS into an app folder (to reduce the diff) and rely on exports to resolve correctly

[mansona]

Context

We want to remove any diff between the Ember app and the rewritten-app so Vite consumes the first directly and the latter can be deactivated.

One of the differences is that files located in my-ember-app/app/* are moved directly to the root of the rewritten-app, because Vite wants the index.html to be at the root. We should find a way to keep the current hierarchy of the Ember app.

In-progress solution

pushed as branch wip-app-namespacing

https://github.com/embroider-build/embroider/tree/wip-app-namespacing