Support ember key in package.json (instead of ember-addon)

[chancancode]

See emberjs/rfcs#823

It was an intention change during the RFC process. We'll probably have to support both for the foreseeable future.

[NullVoxPopuli]

will the two be merged?

if ember-addon specifies stuff in ember, and there are conflicts, should we throw an error?
maybe lint:fix things into ember?

[chancancode]

I think it should probably throw, yeah. "ember-addon" should be considered legacy once this is available. In practice we may be able to go around and change most of the v2 addons we know about pretty quickly.