Fix type inference for autoload scenes while they are being loaded

[tracefree]

Fixes #77643
Fixes #98270

This is a quick and dirty fix to the obscure bug described in the issues above. It's dirty because it relies on the shaky assumption that if the node of the autoload singleton is retrieved as null that means we are in the middle of instantiating that very scene, and because I temporarily store the scene's root node's script path in the value for the autoload, which it is not meant for. So I don't expect this to be merged as-is, just want to kickstart a discussion about possible solutions.

How shaky is the assumption described above? Can it be relied upon after all?

If the approach in principle is okay, what would be a better place to temporarily store the script path?