load packages dynamically, be smart about busting cache

I didn't want to have to direct folks to restart their npm On-Site instance to load new front-page modules.

Worse still, I didn't want to direct people to clear their Redis cache! this would have been horrible!

This pull now:

invalidates the cache automatically if the packages on disk have changed.
loads packages dynamically -- so that no restart will be needed if the packages are updated on the front-page.

npm / npm-explicit-installs