Intellisense in VS Code (both jedi and ms python language server) picks
up all imported names and suggests them with equal priority for
code completion. By adding a leading underscore, intellisense lists
these with a lower priority, so they don't get in the way.
Also rename from builtins to _common while we are touching this to
better reflect the intention of the module (it contains types shared
by multiple modules, not builtin types like int).