Optimization: Cache inspectpack information.

The only really "expensive" thing that `inspectpack` does now is traverse a very limited set of `node_modules` directories to infer the dependency tree. We _could_ cache this if large projects hit problems.

- [ ] Decide if we need to do this. Get a reliable reproduction product to demonstrate the issue and confirm that `inspectpack` is responsible (e.g., the file i/o).
- [ ] Design a caching solution either in `webpack-dashboard` and/or `inspectpack`
- [ ] Handle disk changes in the `node_modules` tree. Could be as simple as documenting "always close and restart the dashboard when updating dependencies".