Allow patches to be defined in the root package

Digging through this project I found that is does not seem to be possible to define a patch for a package in the root package.

This change allows for such:

```json
    "extra": {
      "patches": {
        "vendor/name": {
          "1.2.3": [
            {
              "url": "https://my-domain.com/path/to/my.patch"
            }
          ]
        }
      }
    }
```