Support webpack's config.resolve.modules paths

In our source code, we have imports that do not have paths, and that are not `node-modules`. For example:
```javascript
import 'special-library.lib.js'
```

Currently, when I try to get the dependency tree, this file shows up in the `nonExistent` array and is not resolved.

---

These `lib`files are common library style files we use in our app that we don't want in `node_modules`. This is made possible by [webpack's `config.resolve.modules` option](https://webpack.js.org/configuration/resolve/#resolve-modules):

```javascript
resolve: {
modules: {
'node_modules',
'_libs/' //this tells webpack to search in _libs for imports
}
}
```

How can I get `dependency-tree` to understand my extra webpack modules?