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".