resolve: Desugar empty import groups into synthetic dummy imports

so that they are correctly resolved on 2018 edition

src/test/ui/issues/issue-28388-1.rs

@@ -10,6 +10,6 @@
 
 // Prefix in imports with empty braces should be resolved and checked privacy, stability, etc.
 
-use foo::{}; //~ ERROR cannot find module or enum `foo` in the crate root
+use foo::{}; //~ ERROR unresolved import `foo`
 
 fn main() {}

src/test/ui/issues/issue-28388-1.stderr

@@ -1,9 +1,9 @@
-error[E0578]: cannot find module or enum `foo` in the crate root
+error[E0432]: unresolved import `foo`
   --> $DIR/issue-28388-1.rs:13:5
    |
-LL | use foo::{}; //~ ERROR cannot find module or enum `foo` in the crate root
-   |     ^^^ not found in the crate root
+LL | use foo::{}; //~ ERROR unresolved import `foo`
+   |     ^^^^^^^ no `foo` in the root
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0578`.
+For more information about this error, try `rustc --explain E0432`.