Move to official disabling of tranisitive references

Move to the officially supported method of disabling transitive project
references for VSIX files.

build/Targets/DisableTransitiveReferences.targets

-<?xml version="1.0" encoding="utf-8"?>
-<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-
-  <!-- 
-     Defining this target will disable the new SDK behavior of implicit transitive project
-     references. This is a fine feature in general but the current implementation breaks VSIX 
-     construction by including a number of projects that should not be included. 
-
-     https://github.com/dotnet/sdk/issues/1366
-
-    -->
-  <Target Name="IncludeTransitiveProjectReferences">
-  
-  </Target>
-
-</Project>

build/Targets/Imports.targets

@@ -91,6 +91,9 @@
 
     <IbcMergePath>$(NuGetPackageRoot)\Microsoft.DotNet.IBCMerge\$(MicrosoftDotNetIBCMerge)\lib\net45\ibcmerge.exe</IbcMergePath>
     <MSBuildTargetsFilePath>$(MSBuildToolsPath)\Microsoft.$(MSBuildTargetsLanguageName).targets</MSBuildTargetsFilePath>
+
+    <DisableTransitiveProjectReferences Condition="'$(RoslynProjectType)' == 'Vsix')">true</DisableTransitiveProjectReferences>
+
   </PropertyGroup>
 
   <!-- If the project hasn't configured a ruleset, set a default ruleset. -->
@@ -289,7 +292,6 @@
 
   <Import Project="GenerateAssemblyInfo.targets" Condition="'$(ProjectLanguage)' == 'CSharp' OR '$(ProjectLanguage)' == 'VB'" />
   <Import Project="GenerateInternalsVisibleTo.targets" />
-  <Import Project="DisableTransitiveReferences.targets" Condition="'$(RoslynProjectType)' == 'Vsix' AND '$(RoslynSdkProject)' == 'true'" />
 
   <ItemDefinitionGroup>
     <VSIXSourceItem>