"Required module not found" for subdirectories of node modules.

Discovered this problem while importing `@kadira/storybook`, which made me think it might have been something to do with the `@` symbol at the start of the module name, but I have since replicated it with other requires. Basically when including a module from `node_modules`, you are unable to require anything from a subdirectory of that module:

```js
const submodule = require('my-module/submodule');
^^^^^^^^^^^^^^^^^^^ my-module/submodule. Required module not found
```