Relative paths to GDExtension libraries are "out-of-project" when exporting

[nathanfranke]

Godot version

v4.0.alpha.custom_build [7378dc6]

System information

Arch on 5.18.11-arch1-1

Issue description

I have local paths in my gdextension libraries (so does the official demo):

linux.debug.x86_64 = "bin/libgdextension.linux.debug.x86_64.so"
linux.release.x86_64 = "bin/libgdextension.linux.release.x86_64.so"
android.debug.arm64 = "bin/libgdextension.android.debug.arm64.so"
android.release.arm64 = "bin/libgdextension.android.release.arm64.so"

However when I export, it prints Skipping export of out-of-project library bin/libgdextension.linux.debug.x86_64.so

Only begins_with("res://") is checked.
https://github.com/godotengine/godot/blob/master/editor/plugins/gdextension_export_plugin.h#L74-L77
https://github.com/godotengine/godot/blob/master/editor/plugins/gdextension_export_plugin.h#L133-L136

Steps to reproduce

1. Export project with gdextension

Minimal reproduction project

https://github.com/godotengine/godot-cpp/tree/master/test
https://github.com/nathanfranke/gdextension