Audit command fails if polyfill packages not loaded

https://github.com/composer/composer/issues/10024

I've attempted to workaround this with #15 but it fails if the plugin is installed globally and the polyfill package is not required by the package you are scanning:

```sh
export COMPOSER_HOME=$(mktemp -d)
composer global require cs278/composer-audit:@rc
cd $(mktemp -d)
composer require --no-install cs278/mktemp
composer audit -vvv
```