Add support to "import" a module

[edg-l]

mod test_direct_use {
    mod first {
        mod second {
            fn hello() -> i32 {
                return 2;
            }
        }
    }

    import first.{second};

    pub fn main() -> i32 {
        let mut x: i32 = second::hello();
        return x;
    }
}