@ember/test-waiters should be a non-dev dependency or peerDependency

[gorner]

Our app is currently setup to use ember-try to test against versions of Ember later than the one we're on. Over the weekend, a new version of @ember/test-waiters (restructured as a V2 addon) was released which (for reasons I won't get into right now) started to be included in our ember-try runs, and leads to the following error being raised while booting the app to run tests:

ember-attacher tried to import "@ember/test-waiters" in "ember-attacher/components/attach-popover.js" from addon code, but "@ember/test-waiters" is a devDependency. You may need to move it into dependencies.

I was able to work around this by constraining our app to use @ember/test-waiters v3.1.0, but it does look like ember-attacher is declaring test-waiters under devDependencies when, given that it's accessed in component code and not just in tests, it should be under dependencies or maybe peerDependencies.

Happy to submit a PR, just wanted to get this up quickly to ensure you (and any other users running into this) are aware.

[tylerturdenpants]

Please do!