This PR refactors the internal Ember loader so that it uses the standard
Ember CLI loader instead for modules. This means that modules will be
defined in the main namespace, and importable from there, instead of in
a hidden namespace that only Ember can use.

Notes:

- Code is still loaded and built via `treeForVendor` and included as a
  vendor file. This needs to be the case for the time being for
  bootstrapping.
- Loader code is still included for Node support. If `define` and
  `require` are not already defined, then a backup shim is used instead.
- Modules are now exposed from Ember, but `ember-cli-babel` still
  transpiles them to global references. This unblocks us from being able
  to make all modules work normally, however.
- `require` shim module is no longer defined, we reference `define` and
  `require` as globals instead (which is more accurate). In the future
  we should update this to use Embroider's conventions.
- `__loader` is still exposed on the Ember object, referencing the same
  values as before.