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`.

src/test/ui/resolve/resolve-bad-import-prefix.rs

@@ -17,8 +17,8 @@ use {}; // OK
 use ::{}; // OK
 use m::{}; // OK
 use E::{}; // OK
-use S::{}; //~ ERROR expected module or enum, found struct `S`
-use Tr::{}; //~ ERROR expected module or enum, found trait `Tr`
-use Nonexistent::{}; //~ ERROR cannot find module or enum `Nonexistent` in the crate root
+use S::{}; // FIXME, this and `use S::{self};` should be an error
+use Tr::{}; // FIXME, this and `use Tr::{self};` should be an error
+use Nonexistent::{}; //~ ERROR unresolved import `Nonexistent`
 
 fn main () {}

src/test/ui/resolve/resolve-bad-import-prefix.stderr

@@ -1,24 +1,9 @@
-error[E0577]: expected module or enum, found struct `S`
-  --> $DIR/resolve-bad-import-prefix.rs:20:5
-   |
-LL | use S::{}; //~ ERROR expected module or enum, found struct `S`
-   |     -^^^^
-   |     |
-   |     did you mean `E`?
-
-error[E0577]: expected module or enum, found trait `Tr`
-  --> $DIR/resolve-bad-import-prefix.rs:21:5
-   |
-LL | use Tr::{}; //~ ERROR expected module or enum, found trait `Tr`
-   |     ^^^^^^ not a module or enum
-
-error[E0578]: cannot find module or enum `Nonexistent` in the crate root
+error[E0432]: unresolved import `Nonexistent`
   --> $DIR/resolve-bad-import-prefix.rs:22:5
    |
-LL | use Nonexistent::{}; //~ ERROR cannot find module or enum `Nonexistent` in the crate root
-   |     ^^^^^^^^^^^ not found in the crate root
+LL | use Nonexistent::{}; //~ ERROR unresolved import `Nonexistent`
+   |     ^^^^^^^^^^^^^^^ no `Nonexistent` in the root
 
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
-Some errors occurred: E0577, E0578.
-For more information about an error, try `rustc --explain E0577`.
+For more information about this error, try `rustc --explain E0432`.