[SPARK-13296][SQL] Move UserDefinedFunction into sql.expressions.

[rxin]

This pull request has the following changes:

1. Moved UserDefinedFunction into expressions package. This is more consistent with how we structure the packages for window functions and UDAFs.
2. Moved UserDefinedPythonFunction into execution.python package, so we don't have a random private class in the top level sql package.
3. Move everything in execution/python.scala into the newly created execution.python package.

Most of the diffs are just straight copy-paste.

Test

[rxin]

mima will probably fail

[SparkQA]

Test build #51179 has finished for PR 11181 at commit 1a42847.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

this file is just a simple copy/paste from python.scala.

[SparkQA]

Test build #51180 has finished for PR 11181 at commit 2911532.

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

[SparkQA]

Test build #51198 has finished for PR 11181 at commit 6c8058b.

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

[SparkQA]

Test build #51202 has finished for PR 11181 at commit 66b448d.

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

[rxin]

cc @davies for review

[SparkQA]

Test build #51222 has finished for PR 11181 at commit c268740.

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

[SparkQA]

Test build #51231 has finished for PR 11181 at commit 913bf1b.

• This patch fails from timeout after a configured wait of 250m.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

@davies this did pass tests - just some lingering thing that made the timeout.

[davies]

LGTM

[davies]

BTW, these movement will make us harder to find out the history of the changes, i'd like to not do that if it's not too bad.

[SparkQA]

Test build #2539 has finished for PR 11181 at commit 913bf1b.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

I've merged this in master.