adding outerBuild\InnerBuild structure for generate reference source (#31892)

* Using new custom target from arcade

* removing the outerbuild directory

eng/outerBuild.targets

@@ -0,0 +1,8 @@
+<Project>
+  <Target Name="GenerateReferenceSource" DependsOnTargets="GetProjectWithBestTargetFrameworks">
+    <MSBuild Projects="@(InnerBuildProjectsWithBestTargetFramework)"
+             Targets="GenerateReferenceSource"
+             BuildInParallel="$(BuildInParallel)">
+    </MSBuild>
+  </Target>
+</Project>

eng/testing/outerBuild.targets

@@ -1,17 +1,6 @@
 <Project>
-  <Target Name="Test">
-    <ChooseBestTargetFrameworksTask BuildTargetFrameworks="$(BuildTargetFramework)-$(OSGroup)"
-                                    SupportedTargetFrameworks="$(TargetFrameworks)"
-                                    RuntimeGraph="$(RuntimeGraph)" >
-      <Output TaskParameter="BestTargetFrameworks" ItemName="_BestTargetFramework" />
-    </ChooseBestTargetFrameworksTask>
-
-    <AddTargetFrameworksToProjectTask ProjectName="$(MSBuildProjectFile)"
-                                      BestTargetFrameworks="@(_BestTargetFramework)" >
-      <Output TaskParameter="InnerBuildProjects" ItemName="TestProject" />
-    </AddTargetFrameworksToProjectTask>
-    
-    <MSBuild Projects="@(TestProject)"
+  <Target Name="Test" DependsOnTargets="GetProjectWithBestTargetFrameworks">    
+    <MSBuild Projects="@(InnerBuildProjectsWithBestTargetFramework)"
              Targets="Test"
              BuildInParallel="$(BuildInParallel)">
     </MSBuild>

src/libraries/Directory.Build.targets

@@ -144,6 +144,8 @@
     <Message Text="Generated reference assembly source code: $(_RefSourceFileOutputPath)" />
   </Target>
 
+  <Import Project="$(RepositoryEngineeringDir)outerBuild.targets" Condition="'$(IsCrossTargetingBuild)' == 'true'" />
+
   <!-- Import Packaging targets -->
   <Import Project="$(RepositoryEngineeringDir)packaging.targets" />