Can only populate attrs classes with `_CountingAttr` instances

[redruin1]

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.

[redruin1]

This seems related to #637; the proposed to_field() method would make this trivial.

The utility of this behavior is that it would make it much easier to manipulate the user class before it is passed to attrs plumbing, so you could do things like custom attribute ordering or similar.