Support uid:// paths from external PCK files

[rsubtil]

Closes godotengine/godot-proposals#7181

Because Godot now uses UIDs whenever possible, if these don't exist, warnings are generated. External scenes from PCK files can reference external UIDs not on the original project.

Since the uid_cache.bin is being force-exported anyways, this takes advantage of that fact by appending that information to the UID map.

godot/editor/export/editor_export_platform.cpp

Lines 842 to 845 in 4642448

	String resource_cache_file = ResourceUID::get_cache_file();
	if (FileAccess::exists(resource_cache_file)) {
	files.push_back(resource_cache_file);
	}

[rsubtil]

This was mostly superseeded by #82084, however that PR only fixes the issue for replacing packs.

Therefore, while this can be closed, a solution is still needed for non-replacing packs, which I'll experiment with and see if I can come up with a good approach.