Use JavaPluginExtension to access sourceSet

Author: jmini

src/main/groovy/com/google/protobuf/gradle/ProtobufPlugin.groovy

@@ -192,7 +192,8 @@ class ProtobufPlugin implements Plugin<Project> {
      */
     private Configuration createCompileProtoPathConfiguration(ProtoSourceSet protoSourceSet) {
       String compileProtoConfigName = Utils.getConfigName(protoSourceSet.name, 'compileProtoPath')
-      SourceSet sourceSet = project.sourceSets.getByName(protoSourceSet.name)
+      JavaPluginExtension javaExtension = project.getExtensions().getByType(JavaPluginExtension.class);
+      SourceSet sourceSet = javaExtension.getSourceSets().getByName(protoSourceSet.name)
       Configuration compileConfig =
               project.configurations.getByName(sourceSet.getCompileOnlyConfigurationName())
       Configuration implementationConfig =