Avoid preloading routes in subrequests

Exactly the same thing happens with the cache tag preload subscriber, this might be small enough to do both in one issue.

Nope cache tags properly checks isMainRequest() already.

Also this is wrong in general.

RouteProvider::preloadRoutes() is getting called twice - once with <current> and once with system.404, both load the route from cache because the RoutePreloader::onRequest() never gets called.~

One lookup is for the front page cache context, and the other is for redirect destination. So I was seeing something different to what is actually happening. There might be a saving here, but it's not an issue of the preload routes being loaded twice.