Show single exports of resolved types in nav bar

[samselikoff]

If a module exports a single object, show that object under its resolved type in the side navigation.

Example from Mirage's docs: Collection is a single class export from the addon/orm/collection module.

Closes #267

[samselikoff]

@pzuraq @dfreeman Could I get a quick once-over of this when y'all have the time? This makes single-export modules appear a bit nicer in the nav index

[dfreeman]

Probably best to have Chris weigh in if he has the time, but from my somewhat-less-informed perspective this LGTM, particularly the added test coverage 🎉

(My only minor nit is that I'm not sure why NavigationIndexGenerator is a class instead of just a collection of functions since it has no state, but that's really all I have to complain about 😛)

[pzuraq]

👍 This seems good, definitely think that this is a solid default and makes class discoverability much better

[samselikoff]

My only minor nit is that I'm not sure why NavigationIndexGenerator is a class instead of just a collection of functions since it has no state

@dfreeman good question... I don't really have a lot of experience "organizing free functions" (for lack of a better term), so I usually reach for classes since most of my work has been in OOP systems. I find them to be the easiest way to organize my thinking & refactor into small methods, and of course, there's always the chance that you'll add state later. (In Mirage I wrote the json:api graph serializer initially as a set of functions, then as a bunch of methods on a class, then added some state that tracked "has-been-serialized" models & collections, which I found drastically simplified some of the logic).

That being said I know I simply am not equipped to work on systems that are mostly comprised of functions, so those organizational patterns are just not familiar to me. I'm happy to take this opportunity to learn, if you have some pointers for me.

(One of the existing modules changed in this PR had a combination of a class export and free-floating functions as you can see here in master, but I personally find this sort of file hard to grok, since every function is at the same level. I don't know which ones are "higher-level" and "lower-level". And that's one of the reasons I extracted to a class with "the important stuff at the top.")

Again we can merge this now but I figured I'd write this up because it's something I've been interested on getting a better understanding of!