node_modules symlinks local modules only

(in response to: #831)

@hughsk ah, totally overlooked that - thanks for the explanation. for me, the symlinked lib folder represents modules that are truly local to the app (will never be published on npm) and therefore do not have package.json files.

could we have cake and eat it too by only resolving symlinks for files that have no package.json? seems like folks aren't hot on this idea, but it was pretty easy to add so I figured I'd open another pr just in case.

/cc @timoxley @thlorenz