debugger: use requireRepl() to load debugger repl

Currently, the debugger uses `require('repl')` to setup the repl. However, `require.extensions` is not available yet, causing a crash on tab completion of `require('`. This commit uses the `module.requireRepl()` method to bootstrap the repl.

Closes https://github.com/joyent/node/issues/8359.
Same PR on joyent/node -  https://github.com/joyent/node/pull/8784