Improve s:resolve to also look in parent directories

[cwis-public]

Hi there,

I have recently switched one of my Node package to a “monorepo” structure, and noticed “gf” stopped working when trying to open modules contained within the monorepo.

Turned out, s:resolve wasn't exactly working like require(): in Node.JS, if some package isn't found at a point in path, Node will try all the parent folders, one by one:
https://nodejs.org/api/modules.html#modules_loading_from_node_modules_folders

This PR changes s:resolve to mimic how Node.JS works, and also adds a test for this case.

Thanks for your hard work!

Cheers,
cwis