Append missing trailing path to directory for icon/readme

This MSBuild property inconsistent with MSBuildThisFileDirectory, whereas MSBuildProjectDirectory does

Author: kzu

src/Directory.Build.targets

@@ -40,11 +40,11 @@
     <!-- src-level will need explicit inclusion -->
     <None Include="$(MSBuildThisFileDirectory)icon.png" Link="icon.png" Visible="false" 
           Pack="true" PackagePath="%(Filename)%(Extension)"
-          Condition="Exists('$(MSBuildThisFileDirectory)icon.png') and !Exists('$(MSBuildProjectDirectory)icon.png')" />
+          Condition="Exists('$(MSBuildThisFileDirectory)icon.png') and !Exists('$(MSBuildProjectDirectory)\icon.png')" />
 
     <None Include="$(MSBuildThisFileDirectory)readme.md" Link="readme.md"  
           Pack="true" PackagePath="%(Filename)%(Extension)"
-          Condition="Exists('$(MSBuildThisFileDirectory)readme.md') and !Exists('$(MSBuildProjectDirectory)readme.md')" />
+          Condition="Exists('$(MSBuildThisFileDirectory)readme.md') and !Exists('$(MSBuildProjectDirectory)\readme.md')" />
   </ItemGroup>
 
   <!-- Microsoft.NET.Sdk\targets\Microsoft.NET.DefaultAssemblyInfo.targets does this and is imported