Flatten the synapse.rest.client package (#10600)

changelog.d/10600.misc

synapse/app/generic_worker.py

synapse/handlers/auth.py

synapse/rest/__init__.py

synapse/rest/admin/users.py

synapse/rest/client/__init__.py

synapse/rest/client/_base.py

synapse/rest/client/account.py

synapse/rest/client/account_data.py

synapse/rest/client/account_validity.py

synapse/rest/client/auth.py

synapse/rest/client/capabilities.py

synapse/rest/client/devices.py

synapse/rest/client/directory.py

synapse/rest/client/events.py

synapse/rest/client/filter.py

synapse/rest/client/groups.py

synapse/rest/client/initial_sync.py

synapse/rest/client/keys.py

synapse/rest/client/knock.py