Extract phpstan extension config into its own file

phpstan/config.neon

@@ -4,6 +4,7 @@ includes:
     - ../vendor/phpstan/phpstan-strict-rules/rules.neon
     - ../vendor/phpstan/phpstan-symfony/extension.neon
     - ../vendor/phpstan/phpstan-symfony/rules.neon
+    - ./rules.neon # Composer-specific PHPStan extensions, can be reused by third party packages by including 'vendor/composer/composer/phpstan/rules.neon' in your phpstan config
     - ./baseline.neon
     - ./ignore-by-php-version.neon.php
 
@@ -58,9 +59,3 @@ parameters:
         - Composer\Composer::VERSION
         - Composer\Composer::RELEASE_DATE
         - Composer\Composer::SOURCE_VERSION
-
-services:
-    -
-        class: Composer\PHPStan\ConfigReturnTypeExtension
-        tags:
-            - phpstan.broker.dynamicMethodReturnTypeExtension

phpstan/rules.neon

@@ -0,0 +1,10 @@
+# Composer-specific PHPStan extensions
+#
+# These can be reused by third party packages by including 'vendor/composer/composer/phpstan/rules.neon'
+# in your phpstan config
+
+services:
+    -
+        class: Composer\PHPStan\ConfigReturnTypeExtension
+        tags:
+            - phpstan.broker.dynamicMethodReturnTypeExtension