BuiltinCommands.jsm was huge to avoid slowing things down by having many
modules loading.
To avoid splitting it up from slowing things down we want to delay loading
commands. Create [add|remove]ItemsByModule to allow us to lazily add modules,
and convert all command modules to use this.
Then break up BuiltinCommands into a set of files, for each command, and do
some refactoring to use JS files rather than JSMs and use "use strict".
