Replace Symbol Source

[thargy]

As the current implementation tries to deploy to defunct symbol servers we should change to using GitHub SourceLink.

[josh-green]

We should be looking to switch to the new .snupkg format:
https://docs.microsoft.com/en-us/nuget/create-packages/symbol-packages-snupkg#nugetorg-symbol-package-constraints

Decent blog post on making the switch here:
https://blog.nuget.org/20181116/Improved-debugging-experience-with-the-NuGet-org-symbol-server-and-snupkg.html

[thargy]

James Newton-King seems to suggest symbol packages are currently 'useless' (https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/sourcelink)

Ordinarily, you'd publish symbol files in a symbol package. Currently the main public host for symbol packages doesn't support the portable symbol files (*.pdb) created by SDK-style projects, and symbol packages aren't useful.

However, NuGet/Home#6104 (comment) shows that there's still a lot of confusion and discussion ongoing so we should monitor carefully to see where it ends up! The discussion is quite interesting and continues in dotnet/docs#9110.

Overall, JNK has done some pretty good investigation for the guidance and is definitely an authority of package publication so we should follow the published guidance, whatever it ends up being. I suspect it will end up as 'do all the above'.