ExternalMethod: revert getFunction() to return a function as its name suggests

Commit b45914fa changed the return value of
getFunction() to return more information. But it's easy to preserve the
previous API for any code that would use it, and anyway, it became misnamed.

product/ERP5Type/patches/ExternalMethod.py

@@ -25,13 +25,16 @@ class _(PatchClass(ExternalMethod)):
 
     @property
     def func_defaults(self):
-        return self.getFunction()[1]
+        return self._getFunction()[1]
 
     @property
     def func_code(self):
-        return self.getFunction()[2]
+        return self._getFunction()[2]
 
     def getFunction(self, reload=False):
+        return self._getFunction(reload)[0]
+
+    def _getFunction(self, reload=False):
         try:
             component_module = __import__(
                 'erp5.component.extension.' + self._module,
@@ -92,7 +95,7 @@ class _(PatchClass(ExternalMethod)):
         """
         self.checkGuard(True)
 
-        _f = self.getFunction()
+        _f = self._getFunction()
         __traceback_info__ = args, kw, _f[1]
 
         if _f[3]: