minor #16839 [PropertyAccess] minor: constants as internal and removed unused var (dunglas)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #16839).

Discussion
----------

[PropertyAccess] minor: constants as internal and removed unused var

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

6c9bb86 [PropertyAccess] minor: constants as internal and removed unused var

src/Symfony/Component/PropertyAccess/PropertyAccessor.php

@@ -21,18 +21,69 @@ use Symfony\Component\PropertyAccess\Exception\UnexpectedTypeException;
  */
 class PropertyAccessor implements PropertyAccessorInterface
 {
+    /**
+     * @internal
+     */
     const VALUE = 0;
+
+    /**
+     * @internal
+     */
     const IS_REF = 1;
+
+    /**
+     * @internal
+     */
     const ACCESS_HAS_PROPERTY = 0;
+
+    /**
+     * @internal
+     */
     const ACCESS_TYPE = 1;
+
+    /**
+     * @internal
+     */
     const ACCESS_NAME = 2;
+
+    /**
+     * @internal
+     */
     const ACCESS_REF = 3;
+
+    /**
+     * @internal
+     */
     const ACCESS_ADDER = 4;
+
+    /**
+     * @internal
+     */
     const ACCESS_REMOVER = 5;
+
+    /**
+     * @internal
+     */
     const ACCESS_TYPE_METHOD = 0;
+
+    /**
+     * @internal
+     */
     const ACCESS_TYPE_PROPERTY = 1;
+
+    /**
+     * @internal
+     */
     const ACCESS_TYPE_MAGIC = 2;
+
+    /**
+     * @internal
+     */
     const ACCESS_TYPE_ADDER_AND_REMOVER = 3;
+
+    /**
+     * @internal
+     */
     const ACCESS_TYPE_NOT_FOUND = 4;
 
     private $magicCall;