Utilise phpstan/extension-installer.

composer.json

@@ -12,6 +12,7 @@
     "require": {
         "php": ">=7.1",
         "jangregor/phpstan-prophecy": "^0.7",
+        "phpstan/extension-installer": "^1.0",
         "phpstan/phpstan": "~0.12.0",
         "phpstan/phpstan-beberlei-assert": "~0.12.0",
         "phpstan/phpstan-doctrine": "~0.12.6",

loose.neon

-includes:
-    - %rootDir%/../phpstan-beberlei-assert/extension.neon
-
 parameters:
     level: 1
 
     excludes_analyse:
-        - %rootDir%/../../vendor
+        - %rootDir%/../../*

phpunit.neon

-includes:
-    - %rootDir%/../phpstan-phpunit/extension.neon
-    - %rootDir%/../phpstan-phpunit/rules.neon
-    - %rootDir%/../../jangregor/phpstan-prophecy/extension.neon

symfony.neon

-includes:
-    - %rootDir%/../phpstan-doctrine/extension.neon
-    - %rootDir%/../phpstan-doctrine/rules.neon
-    - %rootDir%/../phpstan-symfony/extension.neon
-
 parameters:
     excludes_analyse:
         - %rootDir%/../../var