[SPARK-17237][SQL][FOLLOWUP][WIP] Add a qualifier in pretty expressions

[maropu]

What changes were proposed in this pull request?

This pr added a qualifier for column names in pivot aggregations. #16565 wrongly droped a qualifier for aggregated column names (then, it changed the existing behaviour).

// Spark-v2.1
scala> Seq((1, 2)).toDF("id", "v1").createOrReplaceTempView("s")
scala> Seq((1, 2)).toDF("id", "v2").createOrReplaceTempView("t")
scala> val df1 = sql("SELECT * FROM s")
scala> val df2 = sql("SELECT * FROM t")
scala> df1.join(df2, "id" :: Nil).groupBy("id").pivot("id").max("v1", "v2").show
+---+-------------+-------------+                                               
| id|1_max(s.`v1`)|1_max(t.`v2`)|
+---+-------------+-------------+
|  1|            2|            2|
+---+-------------+-------------+

// Master
scala> df1.join(df2, "id" :: Nil).groupBy("id").pivot("id").max("v1", "v2").show
+---+---------+---------+                                                       
| id|1_max(v1)|1_max(v2)|
+---+---------+---------+
|  1|        2|        2|
+---+---------+---------+

// Master with this pr
scala> df1.join(df2, "id" :: Nil).groupBy("id").pivot("id").max("v1", "v2").show
+---+-----------+-----------+                                                       
| id|1_max(s.v1)|1_max(t.v2)|
+---+-----------+-----------+
|  1|          2|          2|
+---+-----------+-----------+

How was this patch tested?

Added tests in DataFramePivotSuite.

[SparkQA]

Test build #78051 has finished for PR 18302 at commit dbe793d.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78074 has finished for PR 18302 at commit a6aacd0.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78099 has finished for PR 18302 at commit b423e6f.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78100 has finished for PR 18302 at commit a8728f6.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #78101 has finished for PR 18302 at commit d5e0eab.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[maropu]

Since this pr changed the existing behaviour, I'll close this for now.