Slightly reorganise the pelita module

[Debilski]

When preparing #828 it was found that it is too easy to accidentally create a circular import structure (especially between utils.py and game.py). Part of it is caused by global variables; part of it by functions that are in use in different places; part of it because modules do (and import) more than what they should.

We should probably move the common functions and global variables to different files and also think about which parts in the modules can be decoupled.

[Debilski]

I can take a look at this when (if) I’m renewing the network messages and exception framework.