Split up namespaces-for-dir

[cldwalker]

Hi,
I just got a commit into lein that uses part of namespaces-in-dir for just a file. The fn in lein is rather hacky as it depends on a private fn. Would you take a pull to move this fn into bultitude and use it from namespaces-in-dir? I think I'd rename the fn to namespace-for-file.

[Raynes]

Sure. If someone needs a piece of code that isn't a) a separate public function or b) a public function, it definitely should be fixed.