config: Add LRU cache to get_swh_backend_* functions

I noticed tests in other SWH packages were really slow to execute
since recently and it turned out that the root cause of these
slowdowns is that pytest fixtures like the swh_storage one end up
calling the get_swh_backend_module from swh.core.config.

As that function reads Python package entry points from disk, this
slows down tests execution as it is called before each test using
a fixture as described above.

So add LRU cache to these functions to restore previous execution
times in SWH package tests.

swh/core/config.py

-# Copyright (C) 2015-2020  The Software Heritage developers
+# Copyright (C) 2015-2024  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 from copy import deepcopy
+from functools import lru_cache
 from itertools import chain
 import logging
 import os
@@ -332,6 +333,7 @@ def load_from_envvar(default_config: Optional[Dict[str, Any]] = None) -> Dict[st
     return cfg
 
 
+@lru_cache()
 def get_swh_backend_module(swh_package: str, cls: str) -> Tuple[str, Optional[type]]:
     entry_points = get_entry_points(group=f"swh.{swh_package}.classes")
     if not entry_points:
@@ -358,6 +360,7 @@ def get_swh_backend_module(swh_package: str, cls: str) -> Tuple[str, Optional[ty
     return entry_point.module, BackendCls
 
 
+@lru_cache()
 def get_swh_backend_from_fullmodule(
     fullmodule: str,
 ) -> Tuple[Optional[str], Optional[str]]: