[gnulib-tool-py] List of modules: save in local file

[Dmitriy Selyutin]

Hi everyone!

I'm now working on section where you get all the available modules from gnulib-tool. What do you think about this strategy?
1. If the GNULibImport executes the first time, get all the modules in usual way (like in func_all_modules() function).

2. Save the list of modules using json Python module. This will help user to save some time in the future. We can even just save it line-by-line.
3. In the future get the list of the modules with json module. If it was saved line-by-line, just split lines of file.

4. If user needs to refresh list of modules, he can delete json file or call update method from GNULibImport class.

This affects the entire gnulib-tool, but I think it may be useful. I think the new modules will be added not very often. What do you think?