Unclear path to v2 format adoption for engines

[runspired]

Engines are both addons and also not at all addons. For them to convert to v2 I think we need roughly

• to determine what the main entry point should be (probably engine.{js/ts})
• determine how to handle allowing build addons similar to an app can have, since ideally the dev experience and availability of addons for engines should match that of an app, not an addon
• determine what is needed to be configured to allow for tree-shaking an engine. Apps aren't supposed to import from engines, so likely we can be pretty aggressive here so long as the same graph traversal is used for the engine and its routes file as we use for an app and it's router file.

[ef4]

A thing we haven't done yet is design a v2 app format. I think engines should be part of that.

I also think that design will lean heavily on ember's adoption of strict templates with no more global resolving. That leaves engines with much less to do.