extruder: Consistently use "extruder" for the primary extruder

No longer allow the primary extruder to be named "extruder0".  This
avoids internal and external confusion between the printer objects and
config section names.

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

klippy/gcode.py

@@ -404,7 +404,10 @@ class GCodeParser:
             heater = self.printer.lookup_object('heater_bed', None)
         elif 'T' in params:
             index = self.get_int('T', params, minval=0)
-            extruder = self.printer.lookup_object('extruder%d' % (index,), None)
+            section = 'extruder'
+            if index:
+                section = 'extruder%d' % (index,)
+            extruder = self.printer.lookup_object(section, None)
             if extruder is not None:
                 heater = extruder.get_heater()
         elif self.extruder is not None: