Add support for ember-cli addon proxy (bundle caching)

[eoneill]

As of ember-cli@3.28, addon instances may be proxied (see ember-cli/ember-cli#9487). This can become a problem when patching (setting/restoring) methods on the addon instance or comparing with the addon __proto__.

There was an initial workaround for setting/restoring the preprocessJs method (see ember-cli/ember-cli#9562). This has proven to be unsustainable as other similar issues were uncovered.

For example, when an addon is proxied, this comparison will always be true:

(this.addonInstance.__proto__ && this.addonInstance[treeName] !== this.addonInstance.__proto__[treeName])

... as the methods on the __proto__ are the original method, while on the (proxied) addon are proxy methods. This particular scenario manifests in empty addon trees for proxied addons.

The fix here is to get a reference to the real addon whenever patching methods or comparing to the __proto__.
A similar solution was implemented in ember-engines (see ember-engines/ember-engines#769).