Import insertion can remove used names from an import

use heapless::pool;

pool!(P: u8);

fn main() {
    P::grow$0(&mut [0; 1024]);
}

Applying the "Import heapless::pool::singleton::Pool" assist to make grow resolve results in this code, which does not compile, because the pool! macro is no longer getting imported:

use heapless::pool::{self, singleton::Pool};

pool!(P: u8);

fn main() {
    P::grow(&mut [0; 1024]);
}

(we also seem to incorrectly import the macro during name resolution)

Tested with heapless 0.7.1

Reproduce the import-assist behavior with the heapless 0.7.1 example, then trace the import insertion and name-resolution paths involved in the assist. Done means applying the assist keeps the pool macro available and the resulting code compiles without incorrectly importing the macro.