Create zip folder on build for extension (#643)

src/common/Microsoft.DotNet.UpgradeAssistant.Abstractions/build/Microsoft.DotNet.UpgradeAssistant.Abstractions.targets

@@ -181,7 +181,17 @@
   </ItemGroup>
 
   <!-- Ensure the extension is published on build -->
-  <Target Name="PublishUpgradeAssistant" AfterTargets="Build" DependsOnTargets="Publish" />
+  <Target Name="PublishUpgradeAssistant" AfterTargets="Build" DependsOnTargets="Publish">
+    <PropertyGroup>
+      <ExtensionOutputDirectory Condition=" '$(ExtensionOutputDirectory)' == '' ">$(OutDir)</ExtensionOutputDirectory>
+      <ExtensionOutputPath>$(ExtensionOutputDirectory)$(AssemblyName).zip</ExtensionOutputPath>
+    </PropertyGroup>
+        <ZipDirectory
+            Overwrite="true"
+            SourceDirectory="$(PublishDir)"
+            DestinationFile="$(ExtensionOutputPath)" />
+        <Message Importance="High" Text="Created Upgrade Assistant Extension: $(ExtensionOutputPath)" />
+  </Target>
 
   <!-- Remove host supplied assemblies -->
   <Target Name="RemoveUpgradeAssistantHostProvidedAssemblies"