Add 'PackageSourceMapping' entries to Nuget.config, since multiple sources are used (#545)

Author: mschofie

Samples/nuget.config

@@ -16,5 +16,16 @@
         <add key="localpackages" value="localpackages" />
         <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     </packageSources>
+
+    <packageSourceMapping>
+        <packageSource key="WinAppSDK-SampleDeps">
+            <package pattern="Microsoft.WindowsAppSDK.*" />
+        </packageSource>
+        <packageSource key="nuget.org">
+            <package pattern="System.*" />
+            <package pattern="Microsoft.*" />
+        </packageSource>
+    </packageSourceMapping>
+
     <disabledPackageSources />
 </configuration>