Add SourceLink

[fgreinacher]

This PR adds SourceLink support to our assemblies/PDBs.

I verified it using sourcelink test as per the documentation and everything looks good.

[fgreinacher]

Any objections on merging this?

[jpreese]

This actually seems really cool. This has always been a big issue at my place of work (the need to debug packages), and we've always had to do some pretty big heroics to get that functionality.

For this, all I'd want to do is test it. Intentionally break something in the library, create a local package, reference said package, and see what the debugging story is like.

[jpreese]

Hm.. I'm no longer able to create a NuGet package when I checkout this branch locally (dotnet pack). The error is quite lengthy, but here's a snippet.

C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: The "Microsoft.Build.Tasks.Git.LocateRepository" task failed unexpectedly.\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: System.TypeInitializationException: The type initializer for 'LibGit2Sharp.Core.NativeMethods' threw an exception. ---> System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.DotNet.PlatformAbstractions, Version=2.1.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'. The system cannot find the file specified.\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at Microsoft.Build.Tasks.Git.RuntimeIdMap.GetNativeLibraryDirectoryName()\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at Microsoft.Build.Tasks.Git.GitLoaderContext.GetNativeLibraryDirectory() in //src/Microsoft.Build.Tasks.Git/GitLoaderContext.cs:line 55\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at Microsoft.Build.Tasks.Git.GitLoaderContext.LoadUnmanagedDll(String unmanagedDllName) in //src/Microsoft.Build.Tasks.Git/GitLoaderContext.cs:line 30\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at System.Runtime.Loader.AssemblyLoadContext.ResolveUnmanagedDll(String unmanagedDllName, IntPtr gchManagedAssemblyLoadContext)\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at LibGit2Sharp.Core.NativeMethods.git_libgit2_init()\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at LibGit2Sharp.Core.NativeMethods.InitializeNativeLibrary()\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: at LibGit2Sharp.Core.NativeMethods..cctor()\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]
C:\Users\jpree.nuget\packages\microsoft.build.tasks.git\1.0.0-beta-63102-01\build\Microsoft.Build.Tasks.Git.targets(20,5): error MSB4018: --- End of inner exception stack trace ---\r [C:\Users\jpree\Desktop\System.IO.Abstractions\System.IO.Abstractions\System.IO.Abstractions.csproj]

[fgreinacher]

Do you have .NET Core SDK 2.1.300? This seems so be the minimum supported version as per dotnet/sourcelink#107