CopyToOutputDirectory stopped working

### Steps to reproduce

1. Enlist into this repo: https://github.com/Microsoft/VSProjectSystem/
2. Open in Visual Studio 2019
samples/CpsExtension/CpsExtension.sln
3. In Solution Explorer, build CpsExtension.Vsix

### Expected behavior
CustomDebugger.xaml file to be copied to this folder:
CpsExtension.Vsix\bin\Debug\net461\BuildSystem\Rules\CustomDebugger.xaml

### Actual behavior
it doesn't get copied

Project file
```xml

Debug
AnyCPU
$(MSBuildThisFileDirectory)bin\
$(MSBuildThisFileDirectory)obj\
$(BaseIntermediateOutputPath)$(Configuration)\
$(BaseOutputPath)$(Configuration)\
CpsExtension

net461
false
false

PreserveNewest

MSBuild
Microsoft\VisualStudio\v15.0\CpsExtension\%(RelativeDir)
false

PreserveNewest

```

MSBuild version: 16.200.19

Open samples/CpsExtension/CpsExtension.sln and inspect the CpsExtension.Vsix project file shown in the report. Reproduce the Visual Studio 2019 build and trace the CopyToOutputDirectory metadata for BuildSystem\Rules\CustomDebugger.xaml. Done means the file appears under bin\Debug\net461\BuildSystem\Rules after building.

Written by the indexing model from the issue text.