utils: Some changes to module functions.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
not exactly, there is a default value for submodules:`module_info = {'name': module_name, 'size': None, 'submodules': []}`, it will be [], so the `module_info.get('submodules')` will be [] or [submodule1, ...]