Add a fake <Source> tag

Author: premun

eng/Version.Details.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
+  <Source Mapping="sdk" Uri="https://github.com/dotnet/dotnet" Sha="79eb6fdadee438b5edbf2c5f142aa26218b195c3" BarId="263481" />
   <ProductDependencies>
     <Dependency Name="Microsoft.TemplateEngine.Abstractions" Version="10.0.100-preview.5.25222.2">
       <Uri>https://github.com/dotnet/templating</Uri>