Cleanup Microsoft.CodeAnalysis.Workspaces packages

[tmat]

We package Microsoft.CodeAnalysis.Workspaces and Microsoft.CodeAnalysis.Workspaces.Desktop in the same package, which is an unusal pattern that's not supported by NuGet Pack and needs a custom NuSpec file. Remove the custom NuSpec once we target netstandard2.0 and merge these two assemblies.

When referencing Microsoft.CodeAnalysis.Workspaces.Desktop.csproj from other project that produce packages we need to do so with PrivateAssets="all" since the actual Microsoft.CodeAnalysis.Workspaces.Common package reference is added via reference to Microsoft.CodeAnalysis.Workspaces.csproj.