fix: restore catalog imports broken by incomplete refactor in dcc35f7

The 0.6.0 release shipped client.py and catalog.py importing from
catalog_manager (and _models) but those modules were never committed.
Reverted dcc35f7 to bring back the hardcoded catalog. The YAML-backed
manager will land in a future minor release with the missing files
added explicitly.

Closes the install-time ModuleNotFoundError that blocks 'import entsoe'
on every consumer.

Author: jsulopzs