This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
Fixes#18029.
There are ways to load assemblies that the search hook has no way of intercepting,
and handling itself. Such as loading from a byte[] in C# code.
We now handle these cases with a fallback assembly_load_hook,
to avoid crashes when this is indeed done.
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
