[SBRP] Placeholder files shouldn't be filtered out for out-of-support TFMs

[ViktorHofer]

https://github.com/dotnet/source-build-reference-packages/blob/b99d40df247e865fb0a9de15bdcfdfa7f550a55e/src/packageSourceGenerator/PackageSourceGeneratorTask/GetPackageItems.cs#L114

Placeholder files (_._) are important to keep, even if the TFM folder it is placed in is already out-of-support and not interesting for SBRP. Example:

<Project>

  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;netcoreapp2.0</TargetFrameworks>
  </PropertyGroup>

  ...

</Project>

In that example, the netcoreapp2.0 placeholder file gets filtered out and the netstandard2.0 assembly is picked by the consumer. That's incorrect in cases where the assembly got moved inbox or where the implementation differs significantly, i.e. for System.Runtime.CompilerServices.Unsafe.