Marshmallow’s Nested fields with a restricted schema steal the full schema’s registry spot

Nobody has claimed this yet.

If we e.g. have a marshmallow Schema like this:

```py
from marshmallow_sqlalchemy import fields, SQLAlchemyAutoSchema

class FooSchema(SQLAlchemyAutoSchema):
parent = fields.Nested("BarSchema", only=("id", "name"))
```

and call `spec.components.schema('Foo', schema=FooSchema)`, then “BarSchema” gets registered as having only the fields `id` and `name`, and subsequent tries to register the real thing fail.

The problem lies here:

https://github.com/marshmallow-code/apispec/blob/d418e3efa27f799ae3dabbdbb00ccd07eb10ce07/src/apispec/ext/marshmallow/openapi.py#L108

As that function somehow doesn’t give us something useful. Also it should first modify names for schemas where `schema.only is not None` before it modifies schemas where `schema.only is None`.

Start at src/apispec/ext/marshmallow/openapi.py around line 108 and reproduce the issue with the FooSchema example using a restricted Nested field. Verify that registering FooSchema does not reserve BarSchema with only the restricted fields, and that a later registration preserves the full schema.

Written by the indexing model from the issue text.