Disable copying ProjectReferences in generator projects (#74897)

Author: ericstj

eng/references.targets

@@ -7,8 +7,8 @@
     <_FindDependencies>false</_FindDependencies>
   </PropertyGroup>
 
-  <!-- Project references shouldn't be copied to the output for reference or source projects. -->
-  <ItemDefinitionGroup Condition="'$(IsSourceProject)' == 'true' or '$(IsReferenceAssemblyProject)' == 'true'">
+  <!-- Project references shouldn't be copied to the output for reference, source, or generator projects. -->
+  <ItemDefinitionGroup Condition="'$(IsSourceProject)' == 'true' or '$(IsReferenceAssemblyProject)' == 'true' or '$(IsGeneratorProject)' == 'true'">
     <ProjectReference>
       <Private>false</Private>
     </ProjectReference>