Add private p2p targets for VSIX project compatibility

Author: AArnott

src/Nerdbank.GitVersioning.Tasks/build/PrivateP2PCaching.proj

@@ -13,7 +13,11 @@
   <!-- This target is called by Clean or Rebuild on a referencing project. -->
   <Target Name="Clean" />
 
-  <!-- This target is called by their respective targets on referencing projects. -->
+  <!-- These targets are called by their respective targets on referencing projects. -->
   <Target Name="Pack" />
   <Target Name="Publish" />
+
+  <!-- These targets are called when the consuming project is a VSIX project. -->
+  <Target Name="BuiltProjectOutputGroup" />
+  <Target Name="SatelliteDllsProjectOutputGroup" />
 </Project>