Fix index routes inside async pathless routes

[agundermann]

Fixes #3852. The problem was that getIndexRoute didn't consider async getChildRoutes when looking for pathless child routes.

[timdorr]

Thanks!

[taion]

That was intentional, BTW – getChildRoutes blocks the transition without showing any feedback, so we didn't want to unconditionally load all async child routes just because the application didn't define an explicit index route.

[agundermann]

Interesting. I had a feeling it was, but went ahead since the issue was labeled a bug and @ryanflorence didn't intervene when commenting.

Anyway, both implementations seem to have their cons considering the use case in #3852. Also, I think this wouldn't even be an issue if #3232 had been implemented, since we could only load pathless routes then.

[taion]

Well, let's say "not entirely unintentional". The consequence of doing this code is that, once you hit a route, if it doesn't actually have an index route, the router is just going to eagerly load all the sets of child routes deeply.

You'd want an API like #3326 if you wanted to handle this in a principled way. The implementation here has the risk of being quite foot-gun-prone.