`__getattribute__` can incorrectly call `build()` on factory instances

[AdamHillier]

Implicit calling of build() is desired behaviour when accessing a field, but the current behaviour is for this to happen always, including e.g. when accessing instance.__component_parent__, as is frequently done internally. This is incorrect behaviour, and causes failures with confusing error messages (and sometimes infinite recursion). The behaviour in question comes from here:

zookeeper/zookeeper/core/component.py

Lines 215 to 220 in c8eaab4

	@functools.wraps(base_wrapped_fn)
	def wrapped_fn(instance, name):
	result = base_wrapped_fn(instance, name)
	if utils.is_factory_instance(result):
	return result.build()
	return result

We need to check that name is a valid field name before calling build() implicitly.

Apologies to @timdebruin for this bug....