detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
Move:
- `src\test\ui\consts\const-nonzero.rs` to `library\core`
- `src\test\ui\consts\ascii.rs` to `library\core`
- `src\test\ui\consts\cow-is-borrowed` to `library\alloc`
Part of #76268