Do not suggets `use _` when an item named `_` exists

[estebank]

When an item has been named _ (notably, const _: () = (); is valid), we incorrectly include it as a valid import suggestion:

rust/tests/ui/resolve/underscore-bindings-disambiguators.stderr

Lines 1 to 17 in c043085

	error[E0432]: unresolved import `X`
	--> $DIR/underscore-bindings-disambiguators.rs:25:5
	|
	LL | use X as Y;
	| -^^^^^
	| |
	| no `X` in the root
	| help: a similar name exists in the module: `_`
	error[E0432]: unresolved import `Z`
	--> $DIR/underscore-bindings-disambiguators.rs:26:5
	|
	LL | use Z as W;
	| -^^^^^
	| |
	| no `Z` in the root
	| help: a similar name exists in the module: `_`

use _ is never valid, so the suggestion shouldn't be emitted.

[ozankenangungor]

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