Update full_game.go changing the package names / path to smoothly follow the tutorial

[daveicm]

following the tutorial the package name is given the path "github.com/alice/checkers" so here should be "github.com/alice/checkers/x/checkers/rules" pointing to correct path of the rules downloaded

[xavierlepretre]

The content pages of the tutorial use alice because that's a generic enough name that students will put their own. All the files of this repo use b9lab, because github.com/b9lab/checkers can be made real at this address eventually.

If a change is to be made, it is to the content pages. For instance, here, it could be emphasised that alice shows on the pages, but if you follow the code links, the package name maybe different.

[daveicm]

I am wondering if we can introduce a new "environmental variable" containing the package name here when we create it and use it later on on the code

[xavierlepretre]

Naah that would be even more confusing.