Duplicate "files" dependencies

If we manually delete "vendor" dir and then exec `pas install`, pas will regenerate autoload.json.
The problem is "files" dependencies will be duplicated

```
{
  "files": [
    "vendor/react/promise/src/React/Promise/functions.php",
   "vendor/react/promise/src/React/Promise/functions.php"
  ],
...
```

And it caused error
