Virtual entrypoint export modules

[BlueCutOfficial]

This PR follows #1957 and fixes #1960. It implements the second part of the issue:

We don't want Embroider to rely on define. Using this method should rather be an implementation detail that belongs to the Ember app. Instead of defining modules directly in the entrypoint.js, we would prefer this file to export modules, then the Ember app imports these modules and owns the define implementation.

• It moves the module definition from Embroider internals to the Ember app.js
• All modules to define now constitute the virtual entrypoint.js default export.

[mansona]

Lets get this merged 🎉 💪

[mansona]

my 2 comments are super minor and I don't think we should block the PR on them (unless you would prefer to address them?)