Fix router to ensure that a route module is only loaded once especially
in relation to the use of preload strategies with delayed or partial
loading.
Add test to check the interaction of PreloadingStrategy and normal
router navigation under differing scenarios.
Checking:
* Prevention of duplicate loading of modules.
related to #26557
* Prevention of duplicate RouteConfigLoad(Start|End) events
related to #22842
* Ensuring preload strategy remains active for submodules if needed
The selected preload strategy should still decide when to load submodules
* Possibility of memory leak with unfinished preload subscription
related to #26557
* Ensure that the stored loader promise is cleared so that subsequent
load will try the fetch again.
* Add error handle error from loadChildren
* Ensure we handle error from with NgModule create
Fixes#26557#22842#26557
PR Close#40389
95ad452e54