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.
Many moz.build files define things that could be defined in parent
moz.build files. Keeping the number of moz.build files low helps with
build times due to less I/O and fewer directories traversed.
This patch eliminates a lot of moz.build files by moving their content
into parent moz.build files.
