Add support for use in nested addons

[samchrisinger]

Purpose

I was getting an error when trying to use ember-toastr as a nested addon:

⏚ ⌂69% [Sam:~/Documents/workspace/ember-osf] feature/node-mixin+(+37/-173)+* 131 ± ember s
target.import is not a function
TypeError: target.import is not a function
    at Class.module.exports.included (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-toastr/index.js:11:18)
    at /Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/models/addon.js:244:32
    at Array.map (native)
    at Class.eachAddonInvoke (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/models/addon.js:242:22)
    at Class.Addon.included (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/models/addon.js:349:8)
    at EmberAddon.<anonymous> (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/broccoli/ember-app.js:433:15)
    at Array.filter (native)
    at EmberAddon.EmberApp._notifyAddonIncluded (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/broccoli/ember-app.js:428:45)
    at EmberAddon.EmberApp [as appConstructor] (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/broccoli/ember-app.js:109:8)
    at new EmberAddon (/Users/Sam/Documents/workspace/ember-osf/node_modules/ember-cli/lib/broccoli/ember-addon.js:38:8)

And made my change based on some reading around and debugger sessions. I've tested this behavior in an addon and in a regular app. All seems OK.

h/t ember-cli/ember-cli#3718 (comment)

[knownasilya]

Ah, thanks. I forgot to add that piece..

[knownasilya]

Published as 1.4.1

[samchrisinger]

👍 thanks!