[SPARK-21091][SQL] Move constraint code into QueryPlanConstraints

## What changes were proposed in this pull request?
This patch moves constraint related code into a separate trait QueryPlanConstraints, so we don't litter QueryPlan with a lot of constraint private functions.

## How was this patch tested?
This is a simple move refactoring and should be covered by existing tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #18298 from rxin/SPARK-21091.