`setter(transform)` requires the type to be public

[Pauan]

Minimal example reproducing the error:

mod foo {
    use typed_builder::TypedBuilder;

    struct Bar;

    #[derive(TypedBuilder)]
    pub struct Foo {
        #[builder(setter(transform = |x: u32| Bar))]
        foo: Bar,
    }
}

use foo::Foo;

fn main() {
    Foo::builder()
        .foo(5)
        .build();
}

That gives the following undesired error:

error: type `Bar` is private
   --> src/lib.rs:18:10
    |
18  |         .build();
    |          ^^^^^ private type

The foo method accepts a u32, not a Bar, so the Bar type shouldn't be exposed, so there shouldn't be any error.

But there is an error. This makes it impossible to use private types with transform.