Microsoft.Spark nuget package shouldn't work on all TFMs

[eerhardt]

The Microsoft.Spark nuget package has a build\Microsoft.Spark.targets file. Since this file isn't contained in a TFM folder, nuget will think this package works on any TFM, even TFMs that aren't compatible with netstandard2.0.

See dotnet/machinelearning#370 for the same issue that came up in ML.NET.

We should move that .targets file into a build\netstandard2.0 folder.