Add Fan interface for SMART devices (#873)

Enables the Fan interface for devices supporting that component.
Currently the only device with a fan is the ks240 which implements it as
a child device. This PR adds a method `get_module` to search the child
device for modules if it is a WallSwitch device type.

kasa/tests/test_smartdevice.py

@@ -16,6 +16,7 @@ from kasa.smart import SmartDevice
 from .conftest import (
     bulb_smart,
     device_smart,
+    get_device_for_fixture_protocol,
 )
 
 
@@ -121,6 +122,24 @@ async def test_update_module_queries(dev: SmartDevice, mocker: MockerFixture):
             spies[device].assert_not_called()
 
 
+async def test_get_modules(mocker):
+    """Test get_modules for child and parent modules."""
+    dummy_device = await get_device_for_fixture_protocol(
+        "KS240(US)_1.0_1.0.5.json", "SMART"
+    )
+    module = dummy_device.get_module("CloudModule")
+    assert module
+    assert module._device == dummy_device
+
+    module = dummy_device.get_module("FanModule")
+    assert module
+    assert module._device != dummy_device
+    assert module._device._parent == dummy_device
+
+    module = dummy_device.get_module("DummyModule")
+    assert module is None
+
+
 @bulb_smart
 async def test_smartdevice_brightness(dev: SmartDevice):
     """Test brightness setter and getter."""