Fix #[import] macro: drop trailing unit to avoid clippy::unused_unit in user code

[colinrozzi]

When an imported function has no return type (e.g. log: func(msg: string)), the macro previously emitted:

fn log(msg: String) -> () {
    let input = ...;
    let result = packr_guest::__import_impl(...);
    ()  // <-- unused unit
}

Both -> () and the trailing () trigger clippy::unused_unit from inside an attribute macro. Users running cargo clippy -- -D warnings (typical CI setup) get errors they can't fix without an #[allow] workaround.

Now we emit nothing for the return clause and drop the result statement when there's no return type. Same fix in both #[import] (module attr) and #[import_from] (package attr).

Verified against hello-actor: clippy is clean with this change, no #[allow(clippy::unused_unit)] needed.