Add lru_cache to module_available

A significant time in
xarray.backends.common.py:AbstractWriteableDataStore.set_variables is
spent on common.py:is_dask_collection as it checks for the presence
of the module dask.

This time becomes significant in the case of many small files.

xarray/namedarray/utils.py

@@ -3,6 +3,7 @@
 import sys
 import warnings
 from collections.abc import Hashable, Iterable, Iterator, Mapping
+from functools import lru_cache
 from typing import TYPE_CHECKING, Any, TypeVar, cast
 
 import numpy as np
@@ -32,6 +33,7 @@
 T = TypeVar("T")
 
 
+@lru_cache
 def module_available(module: str) -> bool:
     """Checks whether a module is installed without importing it.