Fix can't reference main bundles in production mode uniquely

This fix actually makes the problem more apparent: if steal is asked to load a main bundle in production, it will only use the filename component. So `users/list.js` and `tasks/list.js` would both result in looking for `bundles/list.js`. Instead, this keeps the path component, and so the mains will be unique: `users/list.js` => `bundles/users/list.js` and `tasks/list.js` => `bundles/tasks/list.js`. 

This is the corresponding fix to https://github.com/bitovi/steal-tools/pull/54 - that fix is required to have steal-tools build main bundles at the location that this fix expects them to be.