Fix async recursion

[Nayni]

Right, so this is becoming a longer story then I initially thought :)

I actually found a proper way to reproduce the issue reported in the webpack plugin: dividab/tsconfig-paths-webpack-plugin#11 you can find it in my own fork. Basically the problem is that if you let the final resolution be a node_module (which was also the case in that issue) the recursion wasn't handled properly.

It took me a while to actually figure out why this was happening but it ended up being an implicit fallthrough that would basically keep the recursion going. I've put a comment in the code to explain it explicitly.

When I fixed this up, the example I made where I experienced the same error as the issue in the webpack plugin also disapeared.

If you'd like the updated example from my fork for future reference and testing, I'm happy to add it as a PR there.

TL;DR: async code with callbacks is hard :)

[coveralls]

Coverage decreased (-0.4%) to 77.628% when pulling 9685d9a on Nayni:fix-async-recursion into 5a3a57a on dividab:master.

[jonaskello]

async code with callbacks is hard :)

I could not agree more :-). At some point I was thinking to use async/await instead of callbacks. But I wanted max performance for the webpack plugin and I think using callbacks is slightly faster, at least if you are using a version of node where async/await is not supported natively.