Can only populate attrs classes with `_CountingAttr` instances

A user might want to use attributes from an already @defined class to populate a new attrs class. However, _ClassBuilder expects all given attributes to be instances of _CountingAttr instead of Attribute instances, which raises an exception when provided:

import attrs

@attrs.define
class Example:
    a: int

ExampleCopy = attrs.make_class("ExampleCopy", attrs={"a": attrs.fields(Example).a})
# AttributeError: 'Attribute' object has no attribute '_validator'. Did you mean: 'validator'?

@attrs.define(these={"a": attrs.fields(Example).a})
class ExampleCopy:
    pass
# AttributeError: 'Attribute' object has no attribute '_validator'. Did you mean: 'validator'?

I would either expect _ClassBuilder to handle both kinds of attributes, or there be an officially sanctioned function allowing you to convert an Attribute to a _CountingAttr for cases like this. You can write this conversion function manually, something like:

def convert_to_countingattr(attr):
    """
    Convert an `Attribute` instance to an equivalent `_CountingAttr` instance.
    """
    return attrs.field(**{
        slot: getattr(attr, slot) 
        for slot in attr.__slots__ 
        if slot not in {"name", "eq_key", "order_key", "inherited"}
    })

But this feels janky and prone to breakage.

Start with the _ClassBuilder path used by attrs.make_class and attrs.define, and reproduce the examples with attrs.fields(Example).a. Determine whether existing Attribute instances should be accepted directly or converted through a supported API; done means the reported examples no longer raise the shown AttributeError and the chosen behavior is covered.