[MSBuild] Allow reference paths to be specified for api intersection

<PropertyGroup>
   <IntersectionAssemblyReferencePath>Path1;Path2<IntersectionAssemblyReferencePath>
</PropertyGroup>

Author: mrward

src/Build/NuGet.Build.Packaging.Tasks/ApiIntersect.cs

@@ -15,6 +15,8 @@ public class ApiIntersect : ToolTask
 		[Required]
 		public ITaskItem[] IntersectionAssembly { get; set; }
 
+		public ITaskItem[] ReferencePath { get; set; }
+
 		[Required]
 		public ITaskItem RootOutputDirectory { get; set; }
 
@@ -56,6 +58,12 @@ protected override string GenerateCommandLineCommands()
 				builder.AppendFileNameIfNotNull(assembly.ItemSpec);
 			}
 
+			foreach (var referencePath in ReferencePath.NullAsEmpty())
+			{
+				builder.AppendSwitch("-r");
+				builder.AppendFileNameIfNotNull(referencePath.ItemSpec);
+			}
+
 			return builder.ToString();
 		}
 

src/Build/NuGet.Build.Packaging.Tasks/NuGet.Build.Packaging.targets

@@ -521,6 +521,7 @@ Copyright (c) .NET Foundation. All rights reserved.
 			Framework="%(ReferenceAssemblyFramework.Identity)"
 			RootOutputDirectory="$(IntermediateOutputPath)"
 			IntersectionAssembly="@(IntersectionAssembly)"
+			ReferencePath="$(IntersectionAssemblyReferencePath)"
 			ToolPath="$(ApiIntersectToolPath)"
 			ToolExe="$(ApiIntersectToolExe)">
 			<Output TaskParameter="OutputDirectory" ItemName="_ReferenceAssemblyBuildInfo" />