Optimization: Cache inspectpack information.

[ryan-roemer]

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".