Revert dedupe dependency handling

The changes in substack/node-browserify#880 made the changes in #38 and #39 unnecessary, so I reverted them. They were arguably hacks around improper dependency specification that was resolved as of browserify@5.11.0.

For safety, I added a regression test for the missing dependency in deduped modules. It passes in >=5.11 and fails in <=5.10 with `Error: Cannot find module '4'` when executing `common + c`.

Sorry about the churn on this.