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.

Also added a test for `require('` tab completion in the repl module. Creating a test for tab completion with the debugger running is proving to be a bit painful.

Closes #8359