Avoid mutating module exports.

We should stop mutating the modules exports.

This was needed while ember-cli was using `amdStrict` mode (since it did not use the `_interopRequireDefault` babel helper), and that is being unrolled in https://github.com/ember-cli/ember-cli/pull/6827.

Suggested migration path is:

* Change [makeDefaultExport](https://github.com/ember-cli/loader.js/blob/master/lib/loader/loader.js#L127) to use `Object.defineProperty(exports, 'default,...)` with a getter that issues a deprecation warning in debug builds (avoiding `Object.defineProperty` in prod builds).
* Remove deprecation and method after major version bump to loader.js@5.0.0.