restore the tokenizer directory when missing

Automatically repopulate the tokenizer/ directory with the PHP stub
and the postgresql module, when the directory is missing. This allows
to switch working directories and in particular run the service
from a different maschine then where it was installed.
Users still need to make sure that .env files are set up correctly
or they will shoot themselves in the foot.

See #2515.

test/bdd/steps/nominatim_environment.py

@@ -217,7 +217,7 @@ class NominatimEnvironment:
                     self.db_drop_database(self.api_test_db)
                     raise
 
-        tokenizer_factory.create_tokenizer(self.get_test_config(), init_db=False)
+        tokenizer_factory.get_tokenizer_for_db(self.get_test_config())
 
 
     def setup_unknown_db(self):