[SPARK-19874][BUILD] Hide API docs for org.apache.spark.sql.internal

## What changes were proposed in this pull request?

The API docs should not include the "org.apache.spark.sql.internal" package because they are internal private APIs.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #17217 from zsxwing/SPARK-19874.

project/SparkBuild.scala

@@ -655,6 +655,7 @@ object Unidoc {
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/util/collection")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/catalyst")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/execution")))
+      .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/internal")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/hive/test")))
   }