fix: avoid hint items shadow local item from completions

[CubeSugarCheese]

Relevant Issues

#18398 Don't auto import items when one is already in scope
#17164 Block specific items from being auto-imported

Fixed Problem

Now completions will not hint item that could shadow local item.

Problems that May Arise

Unexpected behavior

Maybe we can auto rename or use long path to disambiguation, and add a option to control the behavior.

[ChayimFriedman2]

This makes the search N*M, I'd be much happier with a hash set.

[CubeSugarCheese]

This makes the search N*M, I'd be much happier with a hash set.

finish.
During refactoring, I notice that Rustc will treat prelude items and built in types different than normal items.
For prelude, allow shadow by use statement. Like std::result::Result::Ok shadow by use anyhow::Ok. This is not what we want.
For built in types, allow same name item. Like str and core::str, it also allow coexist with user defined item. This feature must support.
Finally, I choose to forbid same items and ignore all built in types. This rule may need to be improved.