heaters: Make heater.py an "extras" module

The heater logic is an independent module that does not need to be
treated as part of the "core" klipper code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/heater_generic.py

@@ -5,4 +5,5 @@
 # This file may be distributed under the terms of the GNU GPLv3 license.
 
 def load_config_prefix(config):
-    return config.get_printer().lookup_object('heater').setup_heater(config)
+    pheaters = config.get_printer().try_load_module(config, 'heaters')
+    return pheaters.setup_heater(config)