* [core] add sources to source collection before triggering load
* [test] add testcase for #15514
* [core] also call onSourceLoaded observers when no network request was necessary
The implementation is buggy and not worth maintaining anymore
because performance benefits are not substantial or sometimes
worse. Also, removing it saves about 150 ~ 180 KB in binary size.
Below timings are averages of minimum 5 runs.
```
Device Init launch Average relaunch
s10 1129.8 ms 700 ms
s10 - binary 1346.75 ms 694 ms
Pixel 1692 ms 723 ms
Pixel - binary 1883 ms 1039 ms
Kazam 17948 ms 1339 ms
Kazam - binary 19157 ms 1564 ms
Wiko 2060 ms 1278 ms
Wiko - binary 3876 ms 1136 ms
```
Fixes#14294
