[SQL] Make Strategies a public developer API

Author: Michael Armbrust <michael@databricks.com>

Closes apache#4920 from marmbrus/openStrategies and squashes the following commits:

cbc35c0 [Michael Armbrust] [SQL] Make Strategies a public developer API

sql/core/src/main/scala/org/apache/spark/sql/package.scala

@@ -34,10 +34,13 @@ import org.apache.spark.sql.execution.SparkPlan
 package object sql {
 
   /**
-   * Converts a logical plan into zero or more SparkPlans.
+   * Converts a logical plan into zero or more SparkPlans.  This API is exposed for experimenting
+   * with the query planner and is not designed to be stable across spark releases.  Developers
+   * writing libraries should instead consider using the stable APIs provided in
+   * [[org.apache.spark.sql.sources]]
    */
   @DeveloperApi
-  protected[sql] type Strategy = org.apache.spark.sql.catalyst.planning.GenericStrategy[SparkPlan]
+  type Strategy = org.apache.spark.sql.catalyst.planning.GenericStrategy[SparkPlan]
 
   /**
    * Type alias for [[DataFrame]]. Kept here for backward source compatibility for Scala.