Structured suggestion for extern crate foo when foo isn't resolved in import

When encountering a name in an import that could have come from a crate that wasn't imported, use a structured suggestion to suggest `extern crate foo;` pointing at the right place in the crate.

When encountering `_` in an import, do not suggest `extern crate _;`.

```
error[E0432]: unresolved import `spam`
  --> $DIR/import-from-missing-star-3.rs:2:9
   |
LL |     use spam::*;
   |         ^^^^ maybe a missing crate `spam`?
   |
help: consider importing the `spam` crate
   |
LL + extern crate spam;
   |
```

compiler/rustc_resolve/src/diagnostics.rs

@@ -2026,14 +2026,17 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                         Applicability::MaybeIncorrect,
                     )),
                 )
+            } else if ident.name == kw::Underscore {
+                (format!("`_` is not a valid crate or module name"), None)
             } else if self.tcx.sess.is_rust_2015() {
                 (
                     format!("you might be missing crate `{ident}`"),
                     Some((
-                        vec![],
-                        format!(
-                            "consider adding `extern crate {ident}` to use the `{ident}` crate"
-                        ),
+                        vec![(
+                            self.current_crate_outer_attr_insert_span,
+                            format!("extern crate {ident};\n"),
+                        )],
+                        format!("consider importing the `{ident}` crate"),
                         Applicability::MaybeIncorrect,
                     )),
                 )

compiler/rustc_resolve/src/lib.rs

@@ -1180,6 +1180,10 @@ pub struct Resolver<'a, 'tcx> {
     /// Simplified analogue of module `resolutions` but in trait impls, excluding glob delegations.
     /// Needed because glob delegations exclude explicitly defined names.
     impl_binding_keys: FxHashMap<LocalDefId, FxHashSet<BindingKey>>,
+
+    /// This is the `Span` where an `extern crate foo;` suggestion would be inserted, if `foo`
+    /// could be a crate that wasn't imported. For diagnostics use only.
+    current_crate_outer_attr_insert_span: Span,
 }
 
 /// Nothing really interesting here; it just provides memory for the rest of the crate.
@@ -1342,6 +1346,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         tcx: TyCtxt<'tcx>,
         attrs: &[ast::Attribute],
         crate_span: Span,
+        current_crate_outer_attr_insert_span: Span,
         arenas: &'a ResolverArenas<'a>,
     ) -> Resolver<'a, 'tcx> {
         let root_def_id = CRATE_DEF_ID.to_def_id();
@@ -1525,6 +1530,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             glob_delegation_invoc_ids: Default::default(),
             impl_unexpanded_invocations: Default::default(),
             impl_binding_keys: Default::default(),
+            current_crate_outer_attr_insert_span,
         };
 
         let root_parent_scope = ParentScope::module(graph_root, &resolver);