Fix test-require-cache-without-stat.js

This path adds an additional cache to the module system for caching the
location of previously required modules. Since it is embedded in the
loop that iterates over all require.paths, this patch also handles the
case where require.paths is being modified.

The patch also cleans up some code around it.

See: https://groups.google.com/forum/#!topic/nodejs-dev/QGGlrvLDHVs