Refactor core.internals into core/internals/ dir
Discussed  in #19268, not sure if the time is right for this since it causes some rebase headaches, but may be worth it.

Note that git is of the opinion that the file core/internals.py was moved to core/internals/blocks.py, not sure if that matters for e.g. blame.

I'm gonna hold off on rebasing this until I get an idea of team's interest in this change.