Use the term 'package' instead of 'module'

The packages really are just collections of resources, with the actual
html files and javascript files and (soon iframes) providing the actual
modular architecture.

In addition 'modules' is used a lot in the javascript community to do
with module loaders etc. We want to avoid confusion there.

Rename 'module' to 'package' everywhere. And move the modules/ directory
to pkg/ which is shorter and clearer.

Closes #1310