Add re-exports to use suggestions

In the following example, an inaccessible path is suggested via
`use foo::bar::X;` whereas an accessible public exported path can
be suggested instead.

```
mod foo {
    mod bar {
        pub struct X;
    }
    pub use self::bar::X;
}

fn main() { X; }
```

This fixes the issue.

src/librustc_resolve/diagnostics.rs

@@ -643,22 +643,24 @@ impl<'a> Resolver<'a> {
         let not_local_module = crate_name.name != kw::Crate;
         let mut worklist =
             vec![(start_module, Vec::<ast::PathSegment>::new(), true, not_local_module)];
+        let mut worklist_via_import = vec![];
 
-        while let Some((in_module, path_segments, accessible, in_module_is_extern)) = worklist.pop()
+        while let Some((in_module, path_segments, accessible, in_module_is_extern)) =
+            match worklist.pop() {
+                None => worklist_via_import.pop(),
+                Some(x) => Some(x),
+            }
         {
             // We have to visit module children in deterministic order to avoid
             // instabilities in reported imports (#43552).
             in_module.for_each_child(self, |this, ident, ns, name_binding| {
-                // avoid imports entirely
-                if name_binding.is_import() && !name_binding.is_extern_crate() {
-                    return;
-                }
-
-                // avoid non-importable candidates as well
+                // avoid non-importable candidates
                 if !name_binding.is_importable() {
                     return;
                 }
 
+                let via_import = name_binding.is_import() && !name_binding.is_extern_crate();
+
                 let child_accessible =
                     accessible && this.is_accessible_from(name_binding.vis, parent_scope.module);
 
@@ -667,6 +669,10 @@ impl<'a> Resolver<'a> {
                     return;
                 }
 
+                if via_import && name_binding.is_possibly_imported_variant() {
+                    return;
+                }
+
                 // collect results based on the filter function
                 // avoid suggesting anything from the same module in which we are resolving
                 if ident.name == lookup_ident.name
@@ -724,7 +730,8 @@ impl<'a> Resolver<'a> {
                         let is_extern = in_module_is_extern || name_binding.is_extern_crate();
                         // add the module to the lookup
                         if seen_modules.insert(module.def_id().unwrap()) {
-                            worklist.push((module, path_segments, child_accessible, is_extern));
+                            if via_import { &mut worklist_via_import } else { &mut worklist }
+                                .push((module, path_segments, child_accessible, is_extern));
                         }
                     }
                 }

src/librustc_resolve/lib.rs

@@ -711,6 +711,13 @@ impl<'a> NameBinding<'a> {
             }
     }
 
+    fn is_possibly_imported_variant(&self) -> bool {
+        match self.kind {
+            NameBindingKind::Import { binding, .. } => binding.is_possibly_imported_variant(),
+            _ => self.is_variant(),
+        }
+    }
+
     // We sometimes need to treat variants as `pub` for backwards compatibility.
     fn pseudo_vis(&self) -> ty::Visibility {
         if self.is_variant() && self.res().def_id().is_local() {

src/test/ui/glob-resolve1.rs

@@ -29,3 +29,7 @@ fn main() {
     foo::<C>(); //~ ERROR: cannot find type `C` in this scope
     foo::<D>(); //~ ERROR: cannot find type `D` in this scope
 }
+
+mod other {
+    pub fn import() {}
+}

src/test/ui/glob-resolve1.stderr

@@ -42,6 +42,11 @@ error[E0425]: cannot find function `import` in this scope
    |
 LL |     import();
    |     ^^^^^^ not found in this scope
+   |
+help: consider importing this function
+   |
+LL | use other::import;
+   |
 
 error[E0412]: cannot find type `A` in this scope
   --> $DIR/glob-resolve1.rs:28:11

src/test/ui/namespace/namespace-mix.stderr

@@ -16,7 +16,7 @@ help: consider importing one of these items instead
    |
 LL | use m2::S;
    |
-LL | use namespace_mix::xm2::S;
+LL | use xm2::S;
    |
 
 error[E0423]: expected value, found type alias `xm1::S`
@@ -39,7 +39,7 @@ help: consider importing one of these items instead
    |
 LL | use m2::S;
    |
-LL | use namespace_mix::xm2::S;
+LL | use xm2::S;
    |
 
 error[E0423]: expected value, found struct variant `m7::V`
@@ -61,7 +61,7 @@ help: consider importing one of these items instead
    |
 LL | use m8::V;
    |
-LL | use namespace_mix::xm8::V;
+LL | use xm8::V;
    |
 
 error[E0423]: expected value, found struct variant `xm7::V`
@@ -83,7 +83,7 @@ help: consider importing one of these items instead
    |
 LL | use m8::V;
    |
-LL | use namespace_mix::xm8::V;
+LL | use xm8::V;
    |
 
 error[E0277]: the trait bound `c::Item: Impossible` is not satisfied

src/test/ui/resolve/issue-21221-2.stderr

@@ -4,7 +4,9 @@ error[E0405]: cannot find trait `T` in this scope
 LL | impl T for Foo { }
    |      ^ not found in this scope
    |
-help: consider importing this trait
+help: consider importing one of these items
+   |
+LL | use baz::T;
    |
 LL | use foo::bar::T;
    |

src/test/ui/resolve/privacy-enum-ctor.stderr

@@ -132,7 +132,7 @@ LL |     let _: E = m::n::Z;
    |            ^
 help: consider importing this enum
    |
-LL | use m::n::Z;
+LL | use m::Z;
    |
 
 error[E0423]: expected value, found enum `m::n::Z`
@@ -165,7 +165,7 @@ LL |     let _: E = m::n::Z::Fn;
    |            ^
 help: consider importing this enum
    |
-LL | use m::n::Z;
+LL | use m::Z;
    |
 
 error[E0412]: cannot find type `Z` in this scope
@@ -183,7 +183,7 @@ LL |     let _: E = m::n::Z::Struct;
    |            ^
 help: consider importing this enum
    |
-LL | use m::n::Z;
+LL | use m::Z;
    |
 
 error[E0423]: expected value, found struct variant `m::n::Z::Struct`
@@ -212,7 +212,7 @@ LL |     let _: E = m::n::Z::Unit {};
    |            ^
 help: consider importing this enum
    |
-LL | use m::n::Z;
+LL | use m::Z;
    |
 
 error[E0603]: enum `Z` is private