module: reduce syscalls during require search

require() now checks that the path exists before searching
further in it.

PR-URL: https://github.com/nodejs/io.js/pull/1920


Reviewed-By: Isaac Z. Schlueter <i@izs.me>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>