Define more empty targets for VSIX projects (#752)

* Define an empty BuiltProjectOutputGroupDependencies target
* Define an empty DebugSymbolsProjectOutputGroup target

Fixes #751

Author: AArnott

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

@@ -20,6 +20,8 @@
   <!-- These targets are called when the consuming project is a VSIX project. -->
   <Target Name="BuiltProjectOutputGroup" />
   <Target Name="SatelliteDllsProjectOutputGroup" />
+  <Target Name="BuiltProjectOutputGroupDependencies" />
+  <Target Name="DebugSymbolsProjectOutputGroup" />
 
   <Import Project="$(NBGV_PrivateP2PAuxTargets)" Condition=" '$(NBGV_PrivateP2PAuxTargets)' != '' " />
 </Project>