Currently, the admin/wizard build relies on the addon build getting
triggered first, so that its `treeForAddon()` hook will be called,
and then it can stash the result on the app's options, which is
super fragile. In Embroider the timing works differently so the
trees end up being `undefined`.
This inverts the logic so that it will be discourse core's build
calling these hooks at a specific timing and return the result
rather than coordinating through the options bag.
```
$ diff dist/assets/admin.js dist-after/assets/admin.js
$ diff dist/assets/wizard.js dist-after/assets/wizard.js
```
