Refactor checker.py

[97littleleaf11]

Currently the checker.py is quite large. Splitting some stateless funcs into submodules might be a good idea.

cc @JukkaL @ilevkivskyi @sobolevn

[97littleleaf11]

Here is some thoughts from @JukkaL :
#12026 (comment)

We can split checker.py by defining a trait/interface that contains commonly used helper methods such as fail and passing the checker instance to another module using this new trait type to avoid an import cycle. We've already split semanal.py in this way (using SemanticAnalyzerInterface), though it's still very big.

The key would be to identify groups of related functionality that make sense to be moved away to a new module. Just moving random bits and pieces to another module doesn't improve the situation much. The semanal_foo modules can perhaps act as some inspiration.

[hauntsaninja]

Closing since this isn't too concrete, but feel free to open PRs