Why can't I copy assemblies like Example.Contracts.dll and CPlugin.Net.Attributes.dll to the plugin output directory?

**This avoids copying the assemblies to the plugin output directory:**
https://github.com/MrDave1999/CPlugin.Net/blob/3d81cb52f65ff0bcf3ce281b6a0b558385f046d3/samples/Plugins/Directory.Build.props#L19-L27

https://github.com/MrDave1999/CPlugin.Net/blob/3d81cb52f65ff0bcf3ce281b6a0b558385f046d3/samples/Plugins/Directory.Build.props#L29-L32