feature symfony#26308 [Config] Introduce BuilderAwareInterface (ro0NL)

This PR was squashed before being merged into the 4.1-dev branch (closes symfony#26308).

Discussion
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[Config] Introduce BuilderAwareInterface

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #...   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

Split `ParentNodeDefinitionInterface` into `BuilderAwareInterface`.

Use case is custom node definition  (extended from VariableNodeDef) with a corresponding prototyped array node.

To set the actual prototype i need the builder at definition level, provided by `ParentNodeDefinitionInterface`. However i don't implement `children()` + `append()`, i solely need the builder scope.

To go after symfony#26297

Commits
-------

1353694 [Config] Introduce BuilderAwareInterface

Author: nicolas-grekas

src/Symfony/Component/Config/Definition/Builder/BuilderAwareInterface.php

@@ -0,0 +1,22 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Config\Definition\Builder;
+
+/**
+ * An interface that can be implemented by nodes which build other nodes.
+ *
+ * @author Roland Franssen <franssen.roland@gmail.com>
+ */
+interface BuilderAwareInterface
+{
+    public function setBuilder(NodeBuilder $builder);
+}

src/Symfony/Component/Config/Definition/Builder/NodeBuilder.php

@@ -179,7 +179,7 @@ public function node($name, $type)
      */
     public function append(NodeDefinition $node)
     {
-        if ($node instanceof ParentNodeDefinitionInterface) {
+        if ($node instanceof BuilderAwareInterface) {
             $builder = clone $this;
             $builder->setParent(null);
             $node->setBuilder($builder);

src/Symfony/Component/Config/Definition/Builder/ParentNodeDefinitionInterface.php

@@ -18,14 +18,12 @@
  *
  * @method NodeDefinition[] getChildNodeDefinitions() should be implemented since 4.1
  */
-interface ParentNodeDefinitionInterface
+interface ParentNodeDefinitionInterface extends BuilderAwareInterface
 {
     /**
      * @return NodeBuilder
      */
     public function children();
 
     public function append(NodeDefinition $node);
-
-    public function setBuilder(NodeBuilder $builder);
 }