Restructure the flat `tractor/` top-level private mods
into (more nested) subpackages:
- `runtime/`: `_runtime`, `_portal`, `_rpc`, `_state`,
`_supervise`
- `spawn/`: `_spawn`, `_entry`, `_forkserver_override`,
`_mp_fixup_main`
- `discovery/`: `_addr`, `_discovery`, `_multiaddr`
Each subpkg `__init__.py` is kept lazy (no eager
imports) to avoid circular import issues.
Also,
- update all intra-pkg imports across ~35 mods to use
the new subpkg paths (e.g. `from .runtime._state`
instead of `from ._state`)
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi