Stop caching packed scenes in GDScript cache (on preload)

[Jordyfel]

Adresses #85081

This code was added in #67714, with the goal of resolving problems with cyclic references is preload().

However, cyclic scene references still don't work, as illustrated by #70985 and other preload related issues.

Being resources, scenes don't support cyclic references, and caching seems to just make the problem appear a little bit later, and make it harder to debug (unlike with scripts, where this should be fine, because get_shallow_script() is designed to deal with the cyclic reference problem).

IMO this problem cannot be solved from outside core resource functionality and the GDScript module should not try. Pinging @adamscott as the original implementer, please do correct me if I have misunderstood the intent of this code.

Most posted issues regarding preload have no reproduction steps and just include already corrupted scenes, which makes it very hard to tell which this change fixes, however it should improve recovery and consistency of behavior as scenes will for sure be reloaded when the script is recompiled.

Bugsquad edit: Fixes #79545 Fixes #84981 Fixes #79840

[adamscott]

Oh, sorry, I did not see your ping! I'll take the time to investigate.

[akien-mga]

I just pinged you 3 min ago so you're not too late :P

[adamscott]

To be honest, if I remember correctly, this was legacy code from when I tried to enable PackedScenes cyclic dependencies. Unfortunately, this idea was shut down, but the code stayed.

The all the tests pass and I even tested a small project. Everything seems to still work.

Great job! Thanks!

[akien-mga]

Thanks!