diagnostics: Don't mention external crates when hitting import errors on crate imports in 2018

compiler/rustc_resolve/src/lib.rs

@@ -2433,8 +2433,10 @@ impl<'a> Resolver<'a> {
                                     Applicability::MaybeIncorrect,
                                 )),
                             )
-                        } else {
+                        } else if self.session.edition() == Edition::Edition2015 {
                             (format!("maybe a missing crate `{}`?", ident), None)
+                        } else {
+                            (format!("could not find `{}` in the crate root", ident), None)
                         }
                     } else if i == 0 {
                         if ident

src/test/ui/editions-crate-root-2015.rs

@@ -18,4 +18,4 @@ mod inner {
 
 fn main() {
 
-}
+}

src/test/ui/editions-crate-root-2018.rs

@@ -5,7 +5,7 @@ mod inner {
         //~^ ERROR failed to resolve: could not find `nonexistant` in the list of imported crates
     }
     fn crate_inner(_: crate::nonexistant::Foo) {
-        //~^ ERROR failed to resolve: maybe a missing crate `nonexistant`?
+        //~^ ERROR failed to resolve: could not find `nonexistant` in the crate root
     }
 
     fn bare_global(_: ::nonexistant) {
@@ -18,4 +18,4 @@ mod inner {
 
 fn main() {
 
-}
+}

src/test/ui/editions-crate-root-2018.stderr

@@ -4,11 +4,11 @@ error[E0433]: failed to resolve: could not find `nonexistant` in the list of imp
 LL |     fn global_inner(_: ::nonexistant::Foo) {
    |                          ^^^^^^^^^^^ could not find `nonexistant` in the list of imported crates
 
-error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
+error[E0433]: failed to resolve: could not find `nonexistant` in the crate root
   --> $DIR/editions-crate-root-2018.rs:7:30
    |
 LL |     fn crate_inner(_: crate::nonexistant::Foo) {
-   |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
+   |                              ^^^^^^^^^^^ could not find `nonexistant` in the crate root
 
 error[E0412]: cannot find crate `nonexistant` in the list of imported crates
   --> $DIR/editions-crate-root-2018.rs:11:25