[MSBuild] Use intersection assembly directory for assembly resolution

By default use the directory containing the intersection assembly when
resolving assemblies using ApiIntersect. This directory is passed to
the ApiIntersect tool using the -r parameter. This behaviour can be
overridden by setting an EnableDefaultIntersectionAssemblyReferencePath
MSBuild property to false. It is true by default.

Author: mrward

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

@@ -517,11 +517,20 @@ Copyright (c) .NET Foundation. All rights reserved.
 		Inputs="@(IntersectionAssembly)"
 		Outputs="@(_ReferenceAssemblyTargetPath)">
 
+		<PropertyGroup>
+			<EnableDefaultIntersectionAssemblyReferencePath Condition="'$(EnableDefaultIntersectionAssemblyReferencePath)' == ''">true</EnableDefaultIntersectionAssemblyReferencePath>
+		</PropertyGroup>
+
+		<ItemGroup>
+			<_IntersectionReferencePath Include="@(IntersectionAssembly->'%(RootDir)%(Directory)')" Condition="'$(EnableDefaultIntersectionAssemblyReferencePath)' == 'true'" />
+			<_IntersectionReferencePath Include="$(IntersectionAssemblyReferencePath)" />
+		</ItemGroup>
+		
 		<ApiIntersect
 			Framework="%(ReferenceAssemblyFramework.Identity)"
 			RootOutputDirectory="$(IntermediateOutputPath)"
 			IntersectionAssembly="@(IntersectionAssembly)"
-			ReferencePath="$(IntersectionAssemblyReferencePath)"
+			ReferencePath="@(_IntersectionReferencePath)"
 			ExcludeType="$(IntersectionExcludeType)"
 			RemoveAbstractTypeMembers="$(IntersectionRemoveAbstractTypeMembers)"
 			ExcludeAssembly="@(IntersectionExcludeAssembly)"