Only suggest imports if not imported.

This commit modifies name resolution error reporting so that if a name
is in scope and has been imported then we do not suggest importing it.

This can occur when we add a label about constructors not being visible
due to private fields. In these cases, we know that the struct/variant
has been imported and we should silence any suggestions to import the
struct/variant.

src/test/ui/issue-42944.rs

@@ -0,0 +1,19 @@
+mod foo {
+    pub struct B(());
+}
+
+mod bar {
+    use foo::B;
+
+    fn foo() {
+        B(()); //~ ERROR expected function, found struct `B` [E0423]
+    }
+}
+
+mod baz {
+    fn foo() {
+        B(()); //~ ERROR cannot find function `B` in this scope [E0425]
+    }
+}
+
+fn main() {}

src/test/ui/issue-42944.stderr

@@ -0,0 +1,20 @@
+error[E0423]: expected function, found struct `B`
+  --> $DIR/issue-42944.rs:9:9
+   |
+LL |         B(()); //~ ERROR expected function, found struct `B` [E0423]
+   |         ^ constructor is not visible here due to private fields
+
+error[E0425]: cannot find function `B` in this scope
+  --> $DIR/issue-42944.rs:15:9
+   |
+LL |         B(()); //~ ERROR cannot find function `B` in this scope [E0425]
+   |         ^ not found in this scope
+help: possible candidate is found in another module, you can import it into scope
+   |
+LL |     use foo::B;
+   |
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0423, E0425.
+For more information about an error, try `rustc --explain E0423`.

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

@@ -2,25 +2,16 @@ error[E0423]: expected value, found struct `Z`
   --> $DIR/privacy-struct-ctor.rs:20:9
    |
 LL |         Z;
-   |         ^ constructor is not visible here due to private fields
-help: a tuple struct with a similar name exists
-   |
-LL |         S;
    |         ^
-help: possible better candidate is found in another module, you can import it into scope
-   |
-LL |     use m::n::Z;
-   |
+   |         |
+   |         constructor is not visible here due to private fields
+   |         help: a tuple struct with a similar name exists: `S`
 
 error[E0423]: expected value, found struct `S`
   --> $DIR/privacy-struct-ctor.rs:33:5
    |
 LL |     S;
    |     ^ constructor is not visible here due to private fields
-help: possible better candidate is found in another module, you can import it into scope
-   |
-LL | use m::S;
-   |
 
 error[E0423]: expected value, found struct `S2`
   --> $DIR/privacy-struct-ctor.rs:38:5