chore: Configure NuGet to avoid additional package sources

Author: jskeet

Google.Api.Generator.Tests/ProtoTests/Directory.Build.props

@@ -1,6 +1,10 @@
 <Project>
   <!-- 
     - This file is present to avoid using the generator's
-    - Directory.Build.props when building the generated test projects.
+    - Directory.Build.props when building the generated test projects,
+    - and to avoid using centralized package versions.
     -->
+  <PropertyGroup>
+    <ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally>
+  </PropertyGroup>
 </Project>

nuget.config

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+  
+  <packageSourceMapping>
+    <packageSource key="nuget.org">
+      <package pattern="*" />
+    </packageSource>
+  </packageSourceMapping>
+</configuration>