fix case when provided trigger path is directory

If trigger is directory and we have permissions to read it, the loaded
will try to load this from it and will fail with IsADirectoryError

tests/ahriman/core/triggers/test_trigger_loader.py

@@ -58,6 +58,15 @@ def test_load_trigger_path(trigger_loader: TriggerLoader, resource_path_root: Pa
     assert trigger_loader._load_trigger(f"{path}.ReportTrigger")
 
 
+def test_load_trigger_path_directory(trigger_loader: TriggerLoader, resource_path_root: Path) -> None:
+    """
+    must raise InvalidExtension if provided import path is directory
+    """
+    path = resource_path_root.parent.parent / "src" / "ahriman" / "core" / "report"
+    with pytest.raises(InvalidExtension):
+        trigger_loader._load_trigger(f"{path}.ReportTrigger")
+
+
 def test_load_trigger_path_not_found(trigger_loader: TriggerLoader) -> None:
     """
     must raise InvalidExtension if file cannot be found