Allow requiring module locally and globally with `fullPaths=true`

This fixes #1020 in conjunction with `watchify` which uses `fullPaths=true`.

**Update:** Turns out this breaks external global access to `require('a')`, e.g. an HTML page with `var a = require('a');` as `a` is now only indexed via `/absolute/path/to/multi_require/a.js`. Related to: https://github.com/substack/watchify/commit/3358625695ad17d4dc3ece639fd7b70c0693d743#commitcomment-9049286

Do you have any ideas on how to restore this behavior from Browserify 4.x/Watchify 0.10.x?