Add sourcelink support (#1574)

* add sourcelink support

* Update sourcelink (2.0.2 -> 2.1.0)

* Add parameter to enable source linking

* tidy up the arguments with an extension method allowing an argument to be conditionally appended

* Add an explicit "test sourcelink" build task and remove the MSBuild one, so we can get some build script output without needing the whole build to be in verbose logging

* run sourcelink test against the NuGet packages

build/Tasks/Default.cs

@@ -1,6 +1,6 @@
 using Cake.Frosting;
 
-[Dependency(typeof(Package))]
+[Dependency(typeof(TestSourceLink))]
 public sealed class Default : FrostingTask<Context>
 {
 }