Merge exports list with package variables in slice JSON.

This implies that all exports are package variables, which made underscore,
jquery, and htmljs (which explicitly assigned to fields on the global variable)
break. We now properly encapsulate these packages (except for window.jQuery,
which we let sneak out because bootstrap wants it).  This means that packages
that want _ need to use underscore, and packages that want $ need to use jquery.
Also, you can't use _ in minimongo $where any more (matching mongod).