Xamarin.Forms-Samples/AppXaml/packages/repositories.config

<?xml version="1.0" encoding="utf-8"?>
<repositories>
  <repository path="..\AppXaml\AppXaml.Droid\packages.config" />
  <repository path="..\AppXaml\AppXaml.iOS\packages.config" />
  <repository path="..\AppXaml\AppXaml.WinPhone\packages.config" />
  <repository path="..\AppXaml\AppXaml\packages.config" />
</repositories>