I have declared my plugin with write permissions to the package directory. After some digging I can find the package from the xcode project plugin working directory. But when trying to alter the package I do not have write permissions to it.
This is not what I would expect as I have the write permissions declared and they are working for spm targets, not for Xcode targets.
That sounds like a bug to me. Are you able to share the package that declares the plugin? In theory there isn't much to declare, but there may be something particular going on.