module: handle NODE_PATH in require('.')

This commit restores the functionality of adding a module's path to
NODE_PATH and requiring it with require('.'). As NODE_PATH was never
intended to be used as a pointer to a module directory (but instead, to
a directory containing directories of modules), this feature is also
being deprecated in turn, to be removed at a later point in time.

PR-URL: https://github.com/iojs/io.js/pull/1363
Fixes: https://github.com/iojs/io.js/issues/1356


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>