Fix #16: Gather all the `:source-paths` in the project file

Fixes #16

* Recursively traverse the project map to collect all the `:source-paths`
* Flatten the result to provide the complete list of `source-dirs`
Indeed, it was not really brilliant to name them like that :smiley: 

P.S.: FYI function `has-doc` as `name` as argument :panda_face: