post transform file cache

Hi folks, I have some tooling built on top of module-deps. Often I already have post transformed source and deps tree information ready to go. It looks like if I provide `opts.cache` it skips reading/parsing source and applying transforms. However, it's unclear to me what the format of a "file object" looks like. Poking through the `walk()` function it looks like I need at least `source`, `package`, and `deps`, so would the format be something like this?

``` js
opts.cache = {
'/User/me/foo.js': {
source: (post transformed source code),
package: ?,
deps: {
'./fooDep1': '/User/me/fooDep1.js',
'../fooDep2': '/User/fooDep2.js'
}
}
};
```

My two questions are:
- I see the transform stream emits other fields too (`file`, `id`, `entry`, etc.) but are those needed in the cache?
- What should the value of `package` field be?
- Any gotchas when it comes to providing cache for files in node_modules?

I'll start running my own tests and exploring, but any guidance would be appreciated. Thanks!