Fix Markdown package by removing extra resources in PRI generation step

Fix thanks to Scott Banning

Author: michael-hawker

Microsoft.Toolkit.Uwp.UI.Controls.Markdown/Microsoft.Toolkit.Uwp.UI.Controls.Markdown.csproj

@@ -46,4 +46,20 @@
     <Message Text="CSFiles: @(GeneratedCSFiles->'&quot;%(Identity)&quot;')" />
     <Exec Command="for %%f in (@(GeneratedCSFiles->'&quot;%(Identity)&quot;')) do echo #pragma warning disable &gt; %%f.temp &amp;&amp; type %%f &gt;&gt; %%f.temp &amp;&amp; move /y %%f.temp %%f &gt; NUL" />
   </Target>
+
+  <!-- 
+    Required workaround for ProjectReference inclusion of the Controls package
+    The UWP project system is including the Controls resources in the pri file because 
+    it doesn't know it'll be an independent package later during packing.
+    Therefore, we need to remove these extra resources in the PRI pipeline so the 
+    Markdown pri file is properly generated and doesn't include duplicate references to Control resources.
+  -->
+  <Target Name="RemoveUnwantedPri" AfterTargets="GetPackagingOutputs">
+    <!--<Message Text="Files Before: @(PackagingOutputs)" Importance="high" />-->
+    <ItemGroup>
+      <PackagingOutputs Remove="@(PackagingOutputs)" Condition="'%(PackagingOutputs.Filename)%(PackagingOutputs.Extension)' == 'Microsoft.Toolkit.Uwp.UI.Controls.pri'" />
+      <PackagingOutputs Remove="@(PackagingOutputs)" Condition="$([System.String]::new('%(PackagingOutputs.TargetPath)').StartsWith('Microsoft.Toolkit.Uwp.UI.Controls\'))" />
+    </ItemGroup>
+    <!--<Message Text="Files After: @(PackagingOutputs)" Importance="high" />-->
+  </Target>
 </Project>