use absolute imports in Python code

Relative imports are no longer officially recommended.

nominatim/clicmd/index.py

@@ -3,8 +3,8 @@ Implementation of the 'index' subcommand.
 """
 import psutil
 
-from ..db import status
-from ..db.connection import connect
+from nominatim.db import status
+from nominatim.db.connection import connect
 
 # Do not repeat documentation of subcommand classes.
 # pylint: disable=C0111