Trim dependencies and generate symbols

[lahma]

I did a quick check as I'm starting my own fork to get netstandard2.0 compatibility and found couple issues that you might want to check out.

• Remove unnecessary dependencies from main library and depend on abstractions
• Mark SourceLink as private asset as it's only used in build and should not create a public dependency
• Also generate symbols that can be uploaded to NuGet to improve debugging experience
• Remove MSTest leftovers from testing project

[JudahGabriel]

Thanks!