Fix how MU blueprints fetches ember-source

[ppcano]

Fix the initial implementation of #8323.

In some cases, the ember-source version was assigned empty because the promise was not fulfilled yet.

I think the test suite did not fail because it may have been using a local ember-source version.

Making locals to return a Promise looks correct based on the locals documentation.

This hook must return an object or a Promise which resolves to an object. The resolved object will be merged with the aforementioned default locals.

I added an assertion that failed previously.

Initially, I realized about this error when adding the feature to the octane-blueprint(the octane blueprints follow this structure), and this error has also been reported at https://github.com/ember-cli/ember-cli/pull/8424/files#r256792430 and #8431

[ppcano]

cc @rwjblue Test passes.