[Feat] Refactor g.extension.py

[NikosAlexandris]

The source code of g.extension.py is too long.
Long ago I started refactoring g.extension.py.
This attempt remained unfinished :-/.

Maybe there is interest to move in more modular structures and easier to read and understand code.

[NikosAlexandris]

There is a lot going on with g.extension but it's still a very long script. Just a reminder that it can become modular and easier to read.