More setting up to run mypy as per #496.

We need `re2` to be a package, not a module, because it appears that
modules can't have `.pyi` files, so munge the module into a package.

Change-Id: I1a268875743390c32c0fb9cd58f6d83a670ce928
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63270
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>