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.
Straight assignments to HierarchicalStringList, which we'll change
EXTRA_JS_MODULES to in a later patch, don't work. This change, in
addition to making things work as expected later on, also is more
consistent with existing practice.
