enable source link for csharp (#4179)

csharp/build_packages.bat

@@ -1,7 +1,7 @@
 @rem Builds Google.Protobuf NuGet packages
 
 dotnet restore src/Google.Protobuf.sln
-dotnet pack -c Release src/Google.Protobuf.sln || goto :error
+dotnet pack -c Release src/Google.Protobuf.sln /p:SourceLinkCreate=true || goto :error
 
 goto :EOF
 

csharp/src/Google.Protobuf/Google.Protobuf.csproj

@@ -17,8 +17,6 @@
     <PackageLicenseUrl>https://github.com/google/protobuf/blob/master/LICENSE</PackageLicenseUrl>
     <RepositoryType>git</RepositoryType>
     <RepositoryUrl>https://github.com/google/protobuf.git</RepositoryUrl>
-    <IncludeSymbols>true</IncludeSymbols>
-    <IncludeSource>true</IncludeSource>
   </PropertyGroup>
 
   <!-- 
@@ -30,4 +28,8 @@
     <TargetFrameworks>netstandard1.0</TargetFrameworks>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="SourceLink.Create.CommandLine" Version="2.7.6" PrivateAssets="All" /> 
+  </ItemGroup>
+
 </Project>