[SPARK-9408] [PYSPARK] [MLLIB] Refactor linalg.py to /linalg

This is based on MechCoder 's PR https://github.com/apache/spark/pull/7731. Hopefully it could pass tests. MechCoder I tried to make minimal changes. If this passes Jenkins, we can merge this one first and then try to move `__init__.py` to `local.py` in a separate PR.

Closes #7731

Author: Xiangrui Meng <meng@databricks.com>

Closes #7746 from mengxr/SPARK-9408 and squashes the following commits:

0e05a3b [Xiangrui Meng] merge master
1135551 [Xiangrui Meng] add a comment for str(...)
c48cae0 [Xiangrui Meng] update tests
173a805 [Xiangrui Meng] move linalg.py to linalg/__init__.py

dev/sparktestsupport/modules.py

@@ -323,7 +323,7 @@ pyspark_mllib = Module(
         "pyspark.mllib.evaluation",
         "pyspark.mllib.feature",
         "pyspark.mllib.fpm",
-        "pyspark.mllib.linalg",
+        "pyspark.mllib.linalg.__init__",
         "pyspark.mllib.random",
         "pyspark.mllib.recommendation",
         "pyspark.mllib.regression",

python/pyspark/sql/types.py

@@ -648,7 +648,7 @@ class UserDefinedType(DataType):
 
     @classmethod
     def fromJson(cls, json):
-        pyUDT = str(json["pyClass"])
+        pyUDT = str(json["pyClass"])  # convert unicode to str
         split = pyUDT.rfind(".")
         pyModule = pyUDT[:split]
         pyClass = pyUDT[split+1:]