Circular requires that work with regular requires do not work with sandboxed-module

[Node.js supports circular requires](https://nodejs.org/api/modules.html#modules_cycles)
The behavior can be odd or unexpected, but it is supported.

In some cases though, this library is loading these circular requires differently, causing them to fail.

I posted an example of such a situation here: https://github.com/thoughtless/circular_require_demo

Another example is the [NPM module](https://www.npmjs.com/package/aws-sdk) `aws-sdk`. It has a circular require between [service.js](https://github.com/aws/aws-sdk-js/blob/08de9516754ea98b88bcc186c8b6ffbf89458502/lib/service.js#L1) and [core.js](https://github.com/aws/aws-sdk-js/blob/08de9516754ea98b88bcc186c8b6ffbf89458502/lib/core.js#L73).
`require("sandboxed-module").require("aws-sdk")` will fail.