General perf improvements for CBT.NuGet (#287)

* Exclude traversal projects from generated solution file

* Only write assets flag file if CBTNuGetGeneratePackageProperties is true

Author: jeffkl

src/CBT.NuGet/Tasks/TraversalNuGetRestore.cs

@@ -162,7 +162,7 @@ private bool TryWriteSolutionFile(ProjectCollection projectCollection)
             {
                 writer.WriteLine("Microsoft Visual Studio Solution File, Format Version 12.00");
 
-                foreach (var project in projectCollection.LoadedProjects)
+                foreach (var project in projectCollection.LoadedProjects.Where(i => !String.Equals(i.GetPropertyValue("IsTraversal"), "true", StringComparison.OrdinalIgnoreCase)))
                 {
                     Uri toUri = new Uri(project.FullPath, UriKind.Absolute);
 

src/CBT.NuGet/build/After.Microsoft.Common.targets

@@ -13,7 +13,7 @@
   <Import Project="$(CBTBuildPackageTargetsFile)" Condition=" '$(CBTEnableImportBuildPackages)' != 'false' And Exists('$(CBTBuildPackageTargetsFile)') "/>
 
   <Target Name="GenerateNuGetAssetFlagFileInsideVisualStudio"
-    Condition=" '$(BuildingInsideVisualStudio)' == 'true' "
+    Condition=" '$(BuildingInsideVisualStudio)' == 'true' And '$(CBTNuGetGeneratePackageProperties)' == 'true'"
     Inputs="$(CBTNuGetRestoreFile)"
     Outputs="$(CBTNuGetAssetsFlagFile)">
 

src/CBT.NuGet/build/CBT.NuGet.props

@@ -181,7 +181,7 @@
   </Target>
 
   <!-- _GenerateRestoreProjectSpec is a from the NuGet.targets new to NuGet 4.x. This target must be in the props chaining in by the cbt\obj\modules\build.props and not in the after.microsoft.common.targets extension as the Microsoft common targets are not pulled in during the NuGet evaluation of the project. -->
-  <Target Name="GenerateNuGetAssetFlagFile" AfterTargets="_GenerateRestoreProjectSpec"  >
+  <Target Name="GenerateNuGetAssetFlagFile" AfterTargets="_GenerateRestoreProjectSpec" Condition=" '$(CBTNuGetGeneratePackageProperties)' == 'true' ">
     <ItemGroup>
       <RestoreAssetsFlagData Remove="@(RestoreAssetsFlagData)"/>
       <RestoreAssetsFlagData Include="ProjectJsonPath">