Don't deploy extensions if they're already installed globally

Rather than failing the build, if Roslyn is installed globally just
don't try to deploy.

Fixes #4189 (https://github.com/dotnet/roslyn/issues/4189)

build/Targets/VSL.Settings.targets

@@ -3,6 +3,13 @@
   <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props"
           Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props') AND '$(MSBuildProjectExtension)' != '.vcxproj'" />
 
+  <PropertyGroup Condition="'$(DevEnvDir)' == ''">
+    <DevEnvDir>$(VSINSTALLDIR)\Common7\IDE</DevEnvDir>
+    <DevEnvDir Condition="'$(VisualStudioVersion)' == '12.0'">$(VS120COMNTOOLS)\..\IDE</DevEnvDir>
+    <DevEnvDir Condition="'$(VisualStudioVersion)' == '14.0'">$(VS140COMNTOOLS)\..\IDE</DevEnvDir>
+    <DevEnvDir>$([System.IO.Path]::GetFullPath('$(DevEnvDir)'))</DevEnvDir>
+  </PropertyGroup>
+
   <PropertyGroup>
     <VisualStudioVersion Condition="'$(VisualStudioVersion)' == ''">12.0</VisualStudioVersion>
     <VisualStudioReferenceMajorVersion Condition="'$(VisualStudioReferenceMajorVersion)' == ''">$(VisualStudioVersion.Substring(0, $(VisualStudioVersion.IndexOf('.'))))</VisualStudioReferenceMajorVersion>
@@ -33,6 +40,10 @@
     <MicrosoftDiaSymReaderNativeVersion>1.1.0-alpha1</MicrosoftDiaSymReaderNativeVersion>
     <NuGetCommandLineVersion>$(NuGetCommandLineAssemblyVersion)</NuGetCommandLineVersion>
     <MicrosoftCompositionVersion>$(MicrosoftCompositionAssemblyVersion)</MicrosoftCompositionVersion>
+
+    <!-- If we still have Roslyn installed, our VSIX-producing packages are not going to install, so let's not even try.
+         OpenSourceDebug overrides this by simple virtue that it's property setting is after the include of this file -->
+    <DeployExtension Condition="Exists('$(DevEnvDir)\CommonExtensions\Microsoft\Roslyn\Language Services\extension.vsixmanifest')">false</DeployExtension>
   </PropertyGroup>
 
   <Choose>
@@ -233,13 +244,6 @@
     <FakeSignToolPath>$(MSBuildThisFileDirectory)..\..\packages\FakeSign.0.9.2\tools\FakeSign.exe</FakeSignToolPath>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(DevEnvDir)' == ''">
-    <DevEnvDir>$(VSINSTALLDIR)\Common7\IDE</DevEnvDir>
-    <DevEnvDir Condition="'$(VisualStudioVersion)' == '12.0'">$(VS120COMNTOOLS)\..\IDE</DevEnvDir>
-    <DevEnvDir Condition="'$(VisualStudioVersion)' == '14.0'">$(VS140COMNTOOLS)\..\IDE</DevEnvDir>
-    <DevEnvDir>$([System.IO.Path]::GetFullPath('$(DevEnvDir)'))</DevEnvDir>
-  </PropertyGroup>
-
   <Choose>
     <When Condition="'$(BuildVersion)' != ''">
       <!-- The user specified a build version number. In that case, we'll use their version number