Investigate how to seperate the getTypeFactory method to getTypeFactoryOfSubchecker and getTypeFactoryOfImmediateSubchecker

[Ao-senXiong]

I think a comment with getTypeFactoryOfSubchecker would be nice that highlights that it only works for immediate subcheckers. Maybe open an issue that we should change getTypeFactoryOfSubchecker to look in all subcheckers and add a getTypeFactoryOfImmediateSubchecker In case that is ever useful?

Originally posted by @wmdietl in #1108 (comment)

[wmdietl]

Same applies to public <T extends SourceChecker> @Nullable T getSubchecker(Class<T> checkerClass) in SourceChecker, which also only goes through the immediate subcheckers.