An option to import traits `as _` when autocompleting

When importing a trait for a method with a code action, rust-analyzer helpfully suggests to import a trait as _. I prefer this style of imports so it’s obvious that the trait name itself isn’t used. Unfortunately, imports-on-completion / flyimports always import the trait name. I would like an option to change that.

I think I could try implementing this? It looks like it just needs a new field in LocatedImport, setting this field in trait_applicable_items() and then using it at some point later where completions are generated. I might need some guidance though, since it’s my first time touching r-a code.

Start by tracing LocatedImport through trait_applicable_items() and the later completion-generation path used by imports-on-completion and flyimports. Confirm where the trait name is selected for an import, then make the new option produce as _ imports; done means the option changes generated trait imports without affecting other imports.