Automatically optimize prefetch_relation to use a single query for single-record relations — WAS: .only fails where .values doesn't

Describe the bug
.only fails where .values succeeds.

To Reproduce

    results = await Mask.all().limit(5).values('name', 'tapeoutprojectname__tapeoutprojectname')

Works

    results = await Mask.all().limit(5).only('name', 'tapeoutprojectname__tapeoutprojectname')

Doesn't and results in:

        if field in self.annotations:
            self._annotations[return_as] = self.annotations[field]
            return

        field_split = field.split("__")
        if field_split[0] in self.model._meta.fetch_fields:
            related_table, related_db_field = self._join_table_with_forwarded_fields(
                model=self.model,
                table=table,
                field=field_split[0],
                forwarded_fields="__".join(field_split[1:]),
            )
            self.query._select_field(related_table[related_db_field].as_(return_as))
            return

>       raise FieldError(f'Unknown field "{field}" for model "{self.model.__name__}"')
E       tortoise.exceptions.FieldError: Unknown field "tapeoutprojectname__tapeoutprojectname" for model "Mask"

../playground/tortoise-orm/tortoise/queryset.py:910: FieldError
============================================================================ 1 failed in 1.69s =============================================================================

Expected behavior
I expected mask objects with tapeoutprojectnames.

Start by reproducing the two Mask queries from the issue and inspect tortoise/queryset.py around line 910, where the nested field raises FieldError. Trace how .values resolves tapeoutprojectname__tapeoutprojectname versus .only, then verify that .only returns Mask objects with the related tapeoutprojectnames and that single-record relations use one query as requested.