Update E0260 to new error format

Updates #35515.
Part of #35233.

r? @jonathandturner

src/librustc_resolve/lib.rs

@@ -3354,7 +3354,11 @@ fn report_conflict(&self,
                 e.span_label(span, &"already imported");
                 e
             },
-            (true, _) | (_, true) => struct_span_err!(self.session, span, E0260, "{}", msg),
+            (true, _) | (_, true) => {
+                let mut e = struct_span_err!(self.session, span, E0260, "{}", msg);
+                e.span_label(span, &format!("`{}` already imported", name));
+                e
+            },
             _ => match (old_binding.is_import(), binding.is_import()) {
                 (false, false) => {
                     let mut e = struct_span_err!(self.session, span, E0428, "{}", msg);

src/test/compile-fail/E0260.rs

@@ -9,8 +9,11 @@
 // except according to those terms.
 
 extern crate collections;
+//~^ NOTE previous import of `collections` here
 
-mod collections { //~ ERROR E0260
+mod collections {
+//~^ ERROR `collections` has already been imported in this module [E0260]
+//~| NOTE `collections` already imported
     pub trait MyTrait {
         fn do_something();
     }