RelBuilder does not, and should not, translate identity projects, even if they rename fields. But when building further relational expressions (e.g. a project) the fields should appear to have their new name.
This requires some tricky implementation. In the RelBuilder, the top RelNode on the stack may have a particular row type (say fields (x, y, z)) yet if will have a frame that indicates that the field names from the user's perspective are (a, b, c).