Pass filtered proto sources to generate proto task

fix: #620

Author: rougsig

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

@@ -248,7 +248,7 @@ class ProtobufPlugin implements Plugin<Project> {
       Provider<ProtobufExtract> extractIncludeProtosTask = setupExtractIncludeProtosTask(
           sourceSet.name, compileProtoPath, sourceSet.compileClasspath)
       Provider<GenerateProtoTask> generateProtoTask = addGenerateProtoTask(
-          sourceSet.name, protoSrcDirSet.sourceDirectories, project.files(extractProtosTask),
+          sourceSet.name, protoSrcDirSet, project.files(extractProtosTask),
           extractIncludeProtosTask) {
         it.sourceSet = sourceSet
         it.doneInitializing()
@@ -321,7 +321,7 @@ class ProtobufPlugin implements Plugin<Project> {
 
       // GenerateProto task, one per variant (compilation unit).
       FileCollection sourceDirs = project.files(project.providers.provider {
-          variant.sourceSets.collect { it.proto.sourceDirectories }
+          variant.sourceSets.collect { it.proto }
       })
       FileCollection extractProtosDirs = project.files(project.providers.provider {
         variant.sourceSets.collect {