reorganize api submodule

Use a directory for the submodule where the __init__ file contains
the public API. This makes it easier to separate public interface
from the internal implementation.

nominatim/api/__init__.py

@@ -0,0 +1,19 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+#
+# This file is part of Nominatim. (https://nominatim.org)
+#
+# Copyright (C) 2023 by the Nominatim developer community.
+# For a full list of authors see the git log.
+"""
+The public interface of the Nominatim library.
+
+Classes and functions defined in this file are considered stable. Always
+import from this file, not from the source files directly.
+"""
+
+# See also https://github.com/PyCQA/pylint/issues/6006
+# pylint: disable=useless-import-alias
+
+from nominatim.api.core import (NominatimAPI as NominatimAPI,
+                                NominatimAPIAsync as NominatimAPIAsync)
+from nominatim.api.status import (StatusResult as StatusResult)