Deprecate getting query parts from QueryBuilder

Author: morozov

UPGRADE.md

@@ -6,6 +6,13 @@ awareness about deprecated code.
 - Use of our low-overhead runtime deprecation API, details:
   https://github.com/doctrine/deprecations/
 
+# Upgrade to 3.8
+
+## Deprecated getting query parts from `QueryBuilder`
+
+The usage of `QueryBuilder::getQueryPart()` and `::getQueryParts()` is deprecated. The query parts
+are implementation details and should not be relied upon.
+
 # Upgrade to 3.6
 
 ## Deprecated not setting a schema manager factory

src/Query/QueryBuilder.php

@@ -1297,6 +1297,8 @@ public function addOrderBy($sort, $order = null)
     /**
      * Gets a query part by its name.
      *
+     * @deprecated The query parts are implementation details and should not be relied upon.
+     *
      * @param string $queryPartName
      *
      * @return mixed
@@ -1309,6 +1311,8 @@ public function getQueryPart($queryPartName)
     /**
      * Gets all query parts.
      *
+     * @deprecated The query parts are implementation details and should not be relied upon.
+     *
      * @return mixed[]
      */
     public function getQueryParts()