Use MSBuild to set NuGet feeds instead of NuGet.config

Directory.Build.props

@@ -5,6 +5,7 @@
 
   <Import Project="version.props" />
   <Import Project="build\dependencies.props" />
+  <Import Project="build\sources.props" />
 
   <PropertyGroup>
     <RepositoryUrl>https://github.com/aspnet/HtmlAbstractions</RepositoryUrl>

NuGet.config

@@ -2,8 +2,6 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="AspNetCore" value="https://dotnet.myget.org/F/aspnetcore-ci-dev/api/v3/index.json" />
+    <!-- Restore sources should be defined in build/sources.props. -->
-    <add key="AspNetCoreTools" value="https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json" />
-    <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>

build/sources.props

@@ -0,0 +1,16 @@
+<Project>
+  <Import Project="$(DotNetRestoreSourcePropsPath)" Condition="'$(DotNetRestoreSourcePropsPath)' != ''"/>
+
+  <PropertyGroup Label="RestoreSources">
+    <RestoreSources>$(DotNetRestoreSources)</RestoreSources>
+    <RestoreSources Condition="'$(DotNetBuildOffline)' != 'true' AND '$(AspNetUniverseBuildOffline)' != 'true' ">
+      $(RestoreSources);
+      https://dotnet.myget.org/F/aspnetcore-ci-dev/api/v3/index.json;
+      https://dotnet.myget.org/F/aspnetcore-tools/api/v3/index.json;
+    </RestoreSources>
+    <RestoreSources Condition="'$(DotNetBuildOffline)' != 'true'">
+      $(RestoreSources);
+      https://api.nuget.org/v3/index.json;
+    </RestoreSources>
+  </PropertyGroup>
+</Project>