Fix dotnet-wpf-test engineering infrastructure

@dotnet-mc-bot commented on [Tue Jul 02 2019](https://github.com/dotnet/core-eng/issues/6860)

There were a set of failures during this build. Here is a summary of these:
* https://dnceng.visualstudio.com/internal/_build/results?buildId=248739
- **Agent:** DDVSOWINAGE316
- **Error log:** E:\A\_work\0\s\.dotnet\sdk\3.0.100-preview7-012512\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(0,0): error NU5017: Cannot create a package that has no dependencies nor content. [E:\A\_work\0\s\packaging\Micros...

@robertborr

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@miguep commented on [Tue Jul 02 2019](https://github.com/dotnet/core-eng/issues/6860#issuecomment-507899553)

I apologize about the noise, this is clearly something wrong with our repo, we are investigating

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@robertborr commented on [Wed Jul 03 2019](https://github.com/dotnet/core-eng/issues/6860#issuecomment-508144388)

dup dotnet/wpf#6837 reclass sourcecode and moving to wpf