WPF workaround for SourceLink and deterministic source paths

Directory.Build.targets

<Project>
  <Target Name="WpfSourceLinkWorkaround" BeforeTargets="InitializeSourceRootMappedPaths" Condition="'$(UseWPF)' == 'true' And $(DeterministicSourcePaths)">
    <!-- WPF causes an error with SourceLink because its build targets create a temporary project without a PackageReference to SourceLink, see https://github.com/dotnet/sourcelink/issues/91,
         causing the @SourceRoot property to be unexpectedly empty for the MapSourceRoot task
        
         For context, see https://github.com/dotnet/roslyn/blob/main/src/Compilers/Core/MSBuildTask/Microsoft.Managed.Core.targets
         and https://github.com/dotnet/roslyn/blob/main/src/Compilers/Core/MSBuildTask/MapSourceRoots.cs
         
         This workaround sets the SourceRoot manually to some deterministic value to keep the promise given by having DeterministicSourcePaths set to true -->
    <Message Text="using deterministic source path workaround for WPF project instead of SourceLink" />
    <ItemGroup>
      <!-- There needs to be at least one SourceRoot defined, its value does not seem to matter as long as it ends with a directory separator -->
      <SourceRoot Include="\" />
    </ItemGroup>
  </Target>
</Project>