Added support for custum vendor dir

I used custom verndor dir,  like this.

composer.json
```
 "config": {
     "vendor-dir": "app/Vendor/Composer"
  }
```
-> install on  "app/Vendor/Composer/apigen/"

in this case, require path is not to be 'vendor/autoloader.php'.
I tryed to fix, I would appreciate it if you merge.

Warm Regards.