Avoid duplicate GenerateProtoTask output dir (#703)

Remove duplicate output source dir that breaks in android in source jar
task.

When using the insertion point, custom plugin needs to use the same
output directory as the code gen output directory from java/javalite.
This causes other tasks to fail if it depends on the output source and
duplicationStrategy is set to `DuplicatesStrategy.FAIL`

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

@@ -571,7 +571,8 @@ public abstract class GenerateProtoTask extends DefaultTask {
   @Internal
   @PackageScope
   Collection<File> getOutputSourceDirectories() {
-    Collection<File> srcDirs = []
+    // insertion point requires the same output source directories as the java plugin. Using a set to avoid duplication.
+    Collection<File> srcDirs = [] as Set
     builtins.each { builtin ->
       File dir = new File(getOutputDir(builtin))
       if (!dir.name.endsWith(".zip") && !dir.name.endsWith(".jar")) {