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)

I don't know if we will be able to enforce includePath from resolvers, there are synthetic modules that might not have an actual filesystem path... sort of hacks but we do this today in one of our resolvers.