Fix build script to explicitly call Pack on the Tasks project

This avoids issues with the IncrementalClean target that runs otherwise
and wipes part of the outputs that are needed for packing.

Author: kzu

build.proj

@@ -25,8 +25,6 @@
     <Solution Include="src\Build\NuGet.Build.Packaging.sln" />
     <Solution Include="src\VisualStudio\NuGet.Packaging.VisualStudio.15\NuGet.Packaging.VisualStudio.15.csproj" />
     <TestProject Include="src\Build\**\*Tests.csproj" />
-    <PackProject Include="src\Build\NuGet.Build.Packaging.Tasks\NuGet.Build.Packaging.Tasks.csproj" />
-    <PackProject Include="src\VisualStudio\NuGet.Packaging.VisualStudio.15\NuGet.Packaging.VisualStudio.15.csproj" />
   </ItemGroup>
 
   <Target Name="All" DependsOnTargets="Build;Test;Package" />
@@ -47,9 +45,10 @@
     <MSBuild Projects="@(Solution)" BuildInParallel="false" Properties="$(CommonBuildProperties);CreateVsixContainer=false;DeployExtension=false" />
   </Target>
 
-  <Target Name="Package" DependsOnTargets="Build">
+  <Target Name="Package">
     <MakeDir Directories="$(Out)" Condition="!Exists('$(Out)')" />
-    <MSBuild Projects="@(PackProject)" Properties="$(CommonBuildProperties);PackOnBuild=true" />
+    <MSBuild Projects="src\Build\NuGet.Build.Packaging.Tasks\NuGet.Build.Packaging.Tasks.csproj" Targets="Pack" Properties="$(CommonBuildProperties);PackOnBuild=true" />
+    <MSBuild Projects="src\VisualStudio\NuGet.Packaging.VisualStudio.15\NuGet.Packaging.VisualStudio.15.csproj" Properties="$(CommonBuildProperties);PackOnBuild=true" />
     <ItemGroup>
       <OutputFile Include="$(Out)\*.nupkg" />
       <OutputFile Include="$(Out)\*.vsix" />