Rename kaya.session_memcache → kaya.session.memcache

- Move src/kaya/session_memcache/ → src/kaya/session/memcache/
- Add pkgutil.extend_path to kaya.session for subpackage namespace support
- Update pyproject.toml version_file path
- Update all import references (tests, READMEs, root README)

packages/kaya-session/src/kaya/session/__init__.py

@@ -1,3 +1,7 @@
+from pkgutil import extend_path
+
+__path__ = extend_path(__path__, __name__)
+
 from ._mixin import SessionMixin
 from ._session import Session
 from ._store import InMemorySessionStore, SessionStore