Add section on package requirements to importSync documentation

[jrjohnson]

Hopefully this will save someone else some time, especially the bit
about ember-auto-import being required.

Refs #1091

[lifeart]

I think it's also could be build-time runtime assert, if importSync is found, we could check for deps

[ef4]

This isn't quite correct.

If the package-being-imported is an ember addon, then you don't need ember-auto-import.

And if the package-doing-the-import is a v2-formatted addon, then you also don't need ember-auto-import, regardless of whether the package-being-imported is an ember addon.

The real rule here is that the argument to importSync should mean the exact same thing as the argument to plain old import. You can't import from NPM without ember-auto-import, so you can't importSync from NPM without ember-auto-import either. It's not that there's a special rule for importSync, it's that it follows the prevailing rules.

[ef4]

So my suggestion for this PR is to word it more as a hint. Something like: "Remember that Ember apps can only import from non-ember-addon NPM packages if they have ember-auto-import, and that rule still applies to importSync".

[jrjohnson]

Thanks for the feedback @ef4. I've updated the language a bit. I didn't use your exact verbiage because I'm not sure this is a "reminder" for most ember devs, it certainly wasn't for me as I don't really know how embroider and ember-auto-import are expected to interact.

[ef4]

Looks good, thanks.

[ef4]

And yes, I appreciate you making the wording more broadly approachable. It's helpful when people contribute to the docs who haven't been stewing so long in the code that they forget what parts are tricky. 😄

[jrjohnson]

Thanks for stewing long enough to make something awesome!