`$(RootNamespace)` caused `__GlobalUsings` error

[Weihan Li]

When we use the --project to exact the global usings and references from project file, it would fail when we're using <Using Include="$(RootNamespace)" /> in our project file

  <ItemGroup>
    <Using Include="$(RootNamespace)" />
  </ItemGroup>

Maybe we could ignore these usings if we could not find a property to replace it in project file