Refactor default_permissions.py into a package

[simonw]

The default_permissions.py file is a bit large and hard to follow: https://github.com/simonw/datasette/blob/68f1179bac991b5e37b99a5482c40134f317c04f/datasette/default_permissions.py. - 494 lines.

[simonw]

Suggested by Claude Opus 4.5 as part of this ad-hoc code review: https://github.com/simonw/research/tree/main/datasette-sql-permissions-review#611-split-default_permissionspy