pkgutil.find_loader() was removed in Python 3.14 (deprecated since
3.12). Replace all calls with importlib.util.find_spec() which is the
recommended alternative.

Both functions return None when the module is not installed, so the
existing None-checks remain correct.

Affected plugins:
- check_free_disk_space.py: checks for pandas, numpy, statsmodels
- monitor_partition_space.py: checks for pandas, numpy