Enforce top-level add-on requirement

[dnalagatla]

Ember-fetch requires the add-on to be a top-level dependency for an application to provide support for Fastboot. This is documented here.

Ember-fetch should enforce the above requirement during the build time of an application if an add-on is using it as a transitive dependency. Recently, ember-data added a dependency on ember-fetch and if the application didn't include ember-fetch dependency, then fastboot failed to load the fetch module.

[dnalagatla]

cc @nlfurniss @xg-wang for visibility.