main: add missing `import importlib.util`

Used in `resolve_collection_argument`. It's implicitly imported by some
other import, but some type checkers don't recognize this.

src/_pytest/main.py

@@ -5,6 +5,7 @@ import dataclasses
 import fnmatch
 import functools
 import importlib
+import importlib.util
 import os
 from pathlib import Path
 import sys