DotNetHost packages are not created during source-build

The subset `host.pkg` is not producing the DotNetHost* NuGet packages. This is because the "Pack" target is getting invoked on the pkgprojs, but pkgprojs expect the "Build" target to be called. Since the "Pack" target is overriden in the Directory.Build.targets to be empty, no one is calling the "Build" target on the pkgprojs.

In an official build, a later subset `packs.tests` comes through and builds `Microsoft.DotNet.CoreSetup.Packaging.Tests.csproj`, which explicitly calls "Build" on the pkgprojs. So official builds still get these packages produced.

In source-build, we are no longer building the `packs.tests` subset, so the packages are not produced.

src/installer/Directory.Build.targets

@@ -15,5 +15,5 @@
   <Import Project="..\..\Directory.Build.targets" />
 
   <!-- Provide default targets which can be hooked onto or overridden as necessary -->
-  <Target Name="Pack" />
+  <Target Name="Pack" DependsOnTargets="Build" />
 </Project>