This patch makes EXTRA_{PP_,}JS_MODULES similar in functionality to
TESTING_JS_MODULES: we indicate the path relative to
$(FINAL_TARGET)/modules with an appropriate hierarchy of paths.
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".
