Add package source mapping

NuGet.config

@@ -84,6 +84,7 @@
       <package pattern="roslyn.diagnostics.analyzers" />
     </packageSource>
     <packageSource key="dotnet-public">
+      <package pattern="azure.core" />
       <package pattern="basic.reference.assemblies" />
       <package pattern="basic.reference.assemblies.*" />
       <package pattern="castle.core" />