Support late disablement by adding a condition on the CollectDeclaredReferences target (#116)

Author: dfederm

src/Package/build/ReferenceTrimmer.targets

@@ -9,7 +9,7 @@
     <CompilerVisibleProperty Include="EnableReferenceTrimmerDiagnostics"/>
   </ItemGroup>
 
-  <Target Name="CollectDeclaredReferences" DependsOnTargets="ResolveAssemblyReferences;PrepareProjectReferences">
+  <Target Name="CollectDeclaredReferences" DependsOnTargets="ResolveAssemblyReferences;PrepareProjectReferences" Condition="'$(EnableReferenceTrimmer)' != 'false'">
     <PropertyGroup>
       <_ReferenceTrimmerDeclaredReferencesFile>$(IntermediateOutputPath)\_ReferenceTrimmer_DeclaredReferences.tsv</_ReferenceTrimmerDeclaredReferencesFile>
       <_ReferenceTrimmerUsedReferencesFile>$(IntermediateOutputPath)\_ReferenceTrimmer_UsedReferences.log</_ReferenceTrimmerUsedReferencesFile>