Find unmet deps in parent directories, searching as node does up as far as the left-most node_modules folder

[studgeek]

See isaacs/npm#2553 for background.

[isaacs]

+1.

I think what really needs to happen here is to move that functionality out of read-installed. There should be a separate standalone requireResolve(id, from, cb) that lives outside of node, and is asynchronous. Then read-installed could just have that figure out which version of what is going to be found, and this could be simplified a great deal.