rename clients to notification_provider_clients

i think it's causing havoc with my attempts to mock stuff in the
`app.clients` directory because it's also accessible at that path. the
name's super vague and doesn't explain what it is anyway

tests/app/dao/test_provider_details_dao.py

@@ -5,7 +5,7 @@ from freezegun import freeze_time
 from sqlalchemy.sql import desc
 
 from app.models import ProviderDetails, ProviderDetailsHistory
-from app import clients
+from app import notification_provider_clients
 from app.dao.provider_details_dao import (
     get_alternative_sms_provider,
     get_provider_details_by_identifier,
@@ -76,7 +76,7 @@ def test_can_get_email_providers(notify_db_session):
 def test_should_not_error_if_any_provider_in_code_not_in_database(restore_provider_details):
     ProviderDetails.query.filter_by(identifier='mmg').delete()
 
-    assert clients.get_sms_client('mmg')
+    assert notification_provider_clients.get_sms_client('mmg')
 
 
 @freeze_time('2000-01-01T00:00:00')