Stop using deprecated distutils.spawn.find_executable

distutils is deprecated and slated for removal. Every import of the
module raises a warning with python3.10.

PEP 632 says to use shutil.which instead.

diffoscope/tools.py

@@ -19,20 +19,18 @@
 import collections
 import functools
 import platform
+import shutil
 
 try:
     import distro
 except ImportError:
     distro = None
 
-from distutils.spawn import find_executable
-
 from .profiling import profile
 from .external_tools import EXTERNAL_TOOLS, REMAPPED_TOOL_NAMES, GNU_TOOL_NAMES
 
-# Memoize calls to ``distutils.spawn.find_executable`` to avoid excessive stat
-# calls
-find_executable = functools.lru_cache()(find_executable)
+# Memoize calls to ``which`` to avoid excessive stat calls
+find_executable = functools.lru_cache()(shutil.which)
 
 # The output of --help and --list-tools will use the order of this dict.
 # Please keep it alphabetized.

tests/utils/tools.py

@@ -24,10 +24,9 @@ import functools
 import importlib.util
 import subprocess
 
-from distutils.spawn import find_executable
 from distutils.version import LooseVersion
 
-from diffoscope.tools import get_package_provider
+from diffoscope.tools import get_package_provider, find_executable
 
 
 def file_version():