repl: fix _debugger by properly proxying repl

The _debugger module uses the internal REPL module, but expects
to receive the userland REPL module. This fixes the breakage that
occurs by proxying the userland REPL module through the internal
module.

It also fixes an unintended in-REPL bug, where require(node-module)
was not resolving correctly.

PR-URL: https://github.com/iojs/io.js/pull/1605


Reviewed-By: Roman Reiss <me@silverwind.io>