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-memcache/pyproject.toml

@@ -51,7 +51,7 @@ strict = true
 
 [tool.setuptools_scm]
 root = "../.."
-version_file = "src/kaya/session_memcache/_version.py"
+version_file = "src/kaya/session/memcache/_version.py"
 
 [tool.setuptools_scm.tag]
 prefix = "release/"