[SPARK-16266][SQL][STREAING] Moved DataStreamReader/Writer from pyspark.sql to...

[SPARK-16266][SQL][STREAING] Moved DataStreamReader/Writer from pyspark.sql to pyspark.sql.streaming

## What changes were proposed in this pull request?

- Moved DataStreamReader/Writer from pyspark.sql to pyspark.sql.streaming to make them consistent with scala packaging
- Exposed the necessary classes in sql.streaming package so that they appear in the docs
- Added pyspark.sql.streaming module to the docs

## How was this patch tested?
- updated unit tests.
- generated docs for testing visibility of pyspark.sql.streaming classes.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #13955 from tdas/SPARK-16266.

python/docs/pyspark.sql.rst

@@ -21,3 +21,9 @@ pyspark.sql.functions module
 .. automodule:: pyspark.sql.functions
     :members:
     :undoc-members:
+
+pyspark.sql.streaming module
+----------------------------
+.. automodule:: pyspark.sql.streaming
+    :members:
+    :undoc-members:

python/pyspark/sql/context.py

@@ -26,7 +26,8 @@ from pyspark import since
 from pyspark.rdd import ignore_unicode_prefix
 from pyspark.sql.session import _monkey_patch_RDD, SparkSession
 from pyspark.sql.dataframe import DataFrame
-from pyspark.sql.readwriter import DataFrameReader, DataStreamReader
+from pyspark.sql.readwriter import DataFrameReader
+from pyspark.sql.streaming import DataStreamReader
 from pyspark.sql.types import Row, StringType
 from pyspark.sql.utils import install_exception_handler
 

python/pyspark/sql/dataframe.py

@@ -33,7 +33,8 @@ from pyspark.storagelevel import StorageLevel
 from pyspark.traceback_utils import SCCallSiteSync
 from pyspark.sql.types import _parse_datatype_json_string
 from pyspark.sql.column import Column, _to_seq, _to_list, _to_java_column
-from pyspark.sql.readwriter import DataFrameWriter, DataStreamWriter
+from pyspark.sql.readwriter import DataFrameWriter
+from pyspark.sql.streaming import DataStreamWriter
 from pyspark.sql.types import *
 
 __all__ = ["DataFrame", "DataFrameNaFunctions", "DataFrameStatFunctions"]

python/pyspark/sql/session.py

@@ -31,7 +31,8 @@ from pyspark.rdd import RDD, ignore_unicode_prefix
 from pyspark.sql.catalog import Catalog
 from pyspark.sql.conf import RuntimeConfig
 from pyspark.sql.dataframe import DataFrame
-from pyspark.sql.readwriter import DataFrameReader, DataStreamReader
+from pyspark.sql.readwriter import DataFrameReader
+from pyspark.sql.streaming import DataStreamReader
 from pyspark.sql.types import Row, DataType, StringType, StructType, _verify_type, \
     _infer_schema, _has_nulltype, _merge_type, _create_converter, _parse_datatype_string
 from pyspark.sql.utils import install_exception_handler