Avoid specifying ReferenceAssembly in definition

Fixes #2165 by never putting a reference to metadata in the ReferenceAssembly field in the first place, so it's never there to get emplaced in a new item escaped and mangle things downstream.

src/Tasks/Microsoft.Common.CurrentVersion.targets

@@ -2049,6 +2049,13 @@ Copyright (C) Microsoft Corporation. All rights reserved.
   <Target Name="FindReferenceAssembliesForReferences"
           DependsOnTargets="ResolveReferences">
     <ItemGroup>
+      <!-- Reference assemblies are not produced in all cases, but it's easier to consume them
+           if this metadatum is always populated. Ensure that it points to the implementation
+           assembly unless already specified. -->
+      <ReferencePath Condition="'%(ReferencePath.ReferenceAssembly)' == ''">
+        <ReferenceAssembly>%(FullPath)</ReferenceAssembly>
+      </ReferencePath>
+
       <ReferencePathWithRefAssemblies Include="@(ReferencePath->'%(ReferenceAssembly)')"
                                       Condition="'$(CompileUsingReferenceAssemblies)' != 'false'">
         <OriginalPath Condition="'%(ReferencePath.Identity)' != '@(ReferencePath->'%(ReferenceAssembly)')'">%(ReferencePath.Identity)</OriginalPath>
@@ -2058,15 +2065,6 @@ Copyright (C) Microsoft Corporation. All rights reserved.
     </ItemGroup>
   </Target>
 
-  <ItemDefinitionGroup>
-    <!-- Reference assemblies are not produced in all cases, but it's easier to consume them
-         if this metadatum is always populated. This item definition ensures that it points
-         to the implementation assembly unless specified. -->
-    <ReferencePath>
-      <ReferenceAssembly>%(FullPath)</ReferenceAssembly>
-    </ReferencePath>
-  </ItemDefinitionGroup>
-
   <!--
     ====================================================================================================