Improves sourcemap infrastructure

* uses `paths` to resolve the relative path to the source in maps
* includes a new property for modules called `sourceFileName` that is meant to be used to generate the right folder structure when debugging using sourcemaps

As a result of this PR, now we can see the right structure when debugging, instead of the current flat list of files, e.g.:

![image](https://cloud.githubusercontent.com/assets/38969/5051146/d7f07180-6c02-11e4-9f3f-1d911a9fc715.png)