Move tname__ to base_shapes to reduce noise in interface classes. (#926)

`tname__` was added to the user facing types. For example, in `models.orm.default`:
```py
class User(base_shapes.User, Named):
    """type default::User"""

    tname__: Literal["default::User"] = Field("default::User", alias="__tname__")
    groups: ComputedMultiLink[UserGroup]
```

This PR would moves `tname__` to `base_shapes.User`.