Install @public and @private via PEP 829 public.start file

This reorganizes the repo into a monorepo of sorts, where the original atpublic project is moved under packages/atpublic and a new atpublic-builtins project is added to packages/atpublic-install.

This latter is there only to install the names into builtins via a PEP 829 .start file which runs public._startup:install() entry point. A parallel .pth file is installed for older Pythons.

Under all of this is the move to hatch workspaces to manage everything.