Auto merge of #57160 - petrochenkov:impice2, r=estebank

resolve: Fix an ICE in import validation

Fixes ICE reported in the comment https://github.com/rust-lang/rust/issues/56596#issuecomment-449866807

src/test/ui/rust-2018/uniform-paths/auxiliary/issue-56596.rs

@@ -0,0 +1 @@
+// Nothing here

src/test/ui/rust-2018/uniform-paths/issue-56596.rs

@@ -0,0 +1,14 @@
+// edition:2018
+// compile-flags: --extern issue_56596
+// aux-build:issue-56596.rs
+
+#![feature(uniform_paths)]
+
+mod m {
+    pub mod issue_56596 {}
+}
+
+use m::*;
+use issue_56596; //~ ERROR `issue_56596` is ambiguous
+
+fn main() {}

src/test/ui/rust-2018/uniform-paths/issue-56596.stderr

@@ -0,0 +1,18 @@
+error[E0659]: `issue_56596` is ambiguous (name vs any other name during import resolution)
+  --> $DIR/issue-56596.rs:12:5
+   |
+LL | use issue_56596; //~ ERROR `issue_56596` is ambiguous
+   |     ^^^^^^^^^^^ ambiguous name
+   |
+   = note: `issue_56596` could refer to an extern crate passed with `--extern`
+   = help: use `::issue_56596` to refer to this extern crate unambiguously
+note: `issue_56596` could also refer to the module imported here
+  --> $DIR/issue-56596.rs:11:5
+   |
+LL | use m::*;
+   |     ^^^^
+   = help: use `crate::issue_56596` to refer to this module unambiguously
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0659`.