Fix Item update in Pack logic due to MSBuild regression

- MSBuild Item update logic within target broke during 17.3-17.4!

  Previous logic referencing direct Item names worked fine before,
  but now needs a proxy/temporary item in order to process the includes.
  Same with the undefined Metadata, previously it returned empty string for
  items with the metadata undefined but now throws error. This may be correct
  behavior for items under target but this difference hinders sharing logic
  within and out of targets. This has a side-effect of needing to specify
  fully qualified name "%(Item.Metadata)" which makes it verbose.

Author: Nirmal4G

eng/Toolkit.GetBuildOutputs.targets

@@ -28,10 +28,10 @@
       <BuildOutputPerTarget Update="@(BuildOutputPerTarget)" TargetFramework="$(TargetFramework)" />
     </ItemGroup>
 
-    <ItemGroup Condition="'$(IsCompilerTargeting)' == 'true'">
-      <BuildOutputPerTarget Update="@(BuildOutputPerTarget)" IsCompilerExtension="true" />
+    <ItemGroup>
       <BuildOutputPerTarget Update="@(BuildOutputPerTarget)" TargetCompiler="$(TargetCompiler)" />
       <BuildOutputPerTarget Update="@(BuildOutputPerTarget)" TargetLanguage="$(TargetLanguage)" />
+      <BuildOutputPerTarget Update="@(BuildOutputPerTarget)" IsCompilerExtension="$(IsCompilerTargeting)" />
     </ItemGroup>
   </Target>
 

eng/Toolkit.Packaging.targets

@@ -78,10 +78,12 @@
 
     <!-- Include the Build Outputs (also for each Custom target, like Roslyn analyzers and generators) in the package -->
     <ItemGroup>
-      <PackageFile Include="@(NonReferencingProjectBuildOutput)">
+      <_NonReferencingProjectPackageFile Include="@(NonReferencingProjectBuildOutput->ClearMetadata())">
         <TargetPath Condition="'%(IsCompilerExtension)' == 'true'">$([System.IO.Path]::Combine('analyzers', 'dotnet', '%(TargetCompiler)', '%(TargetLanguage)'))</TargetPath>
-        <TargetPath Condition="'%(TargetPath)' == ''">$([System.IO.Path]::Combine('tools', '%(TargetFramework)'))</TargetPath>
-      </PackageFile>
+        <TargetPath Condition="'%(IsCompilerExtension)' != 'true'">$([System.IO.Path]::Combine('tools', '%(TargetFramework)'))</TargetPath>
+      </_NonReferencingProjectPackageFile>
+      <PackageFile Include="@(_NonReferencingProjectPackageFile)" />
+      <_NonReferencingProjectPackageFile Remove="@(_NonReferencingProjectPackageFile)" />
     </ItemGroup>
   </Target>