Bundle format fixes

This fixes two major issues which make the current bundle format break:

1. Fixed the `mkdirp.sync()` call from 2ca75d52 to make sure to get the path's dir.
2. Properly handle rebuilds that output to the bundle format by using a safer caching strategy.

The major change here is updating to `es6-module-transpiler@0.6.2` which now has a hook for controlling file IO on the internal `Module` class instances. This allows for a **much safer** _and_ simpler caching strategy for keep re-builds from needing to perform file IO on unchanged modules.
Now only caching the string `src` of the module, instead of trying to re-use the mutated `ast` and derived `imports` and `exports` data structures.

This means that modules will be re-parsed during re-builds, but this caching strategy still prevents needless file IO of unchanged modules.