Simplify how we ship targets files into the C# extension

[davidwengier]

Currently there is a Razor.DesignTime.targets file in the .NET SDK, and we have two pieces of code in the CSharp.DesignTime.targets file in the .NET Project System repo, that allow for various functionality in Visual Studio and in the C# Dev Kit extension.

For the C# Extension, when Dev Kit is not present, we have a somewhat convoluted system to achieve the same thing. We ship copies, or subsets, of those targets files out of this repo with our nuget packages, pull them into the C# extension repo, which then passes command line arguments to Roslyn with the file paths, which in turn sets MSBuild variables in the build host.

We should improve this. Can we remove the duplication of targets files and use the "real" ones in the C# Extension? Can we remove or reduce the number of specific command line arguments that Roslyn needs, and instead have a generic way for extensions to contribute MSBuild variables?