Handle ITaskItem parameter in task runner

[0xced]

Having to convert a ITaskItem[] to a ITaskItem may happen with MSBuild target batching. For example, this happens with the Microsoft.SourceLink.GitHub.GetSourceLinkUrl task where the SourceRoot parameter is somehow translated from what should be a property with metadata to a list of task items.

Without this translation, the following error would occur:

System.ArgumentException: Object of type 'Microsoft.Build.Framework.ITaskItem[]' cannot be converted to type 'Microsoft.Build.Framework.ITaskItem'.

[0xced]

Here's what the SourceRoot looks like in the viewer for the Microsoft.SourceLink.GitHub.GetSourceLinkUrl task.

[KirillOsenkov]

Thanks!