src: fix module search path for preload modules

When the preload module is not a abs/relative path, we should use
the standard search mechanism of looking into the node_modules folders
outwards. The current working directory is deemed to be the 'requiring
module', i.e. parent. The search path starts from cwd outwards.

Fixes: https://github.com/nodejs/io.js/issues/1803
PR-URL: https://github.com/nodejs/io.js/pull/1812


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>