Make --error-format human-annotate-rs handle multiple files

tests/ui/annotate-snippet/auxiliary/other_file.rs

@@ -0,0 +1,6 @@
+pub struct WithPrivateMethod;
+
+impl WithPrivateMethod {
+    /// Private to get an error involving two files
+    fn private_method(&self) {}
+}

tests/ui/annotate-snippet/multiple-files.rs

@@ -0,0 +1,8 @@
+// aux-build:other_file.rs
+// compile-flags: --error-format human-annotate-rs -Z unstable-options
+
+extern crate other_file;
+
+fn main() {
+    other_file::WithPrivateMethod.private_method();
+}

tests/ui/annotate-snippet/multiple-files.stderr

@@ -0,0 +1,11 @@
+error[E0624]: method `private_method` is private
+  --> $DIR/multiple-files.rs:7:35
+   |
+LL |     other_file::WithPrivateMethod.private_method();
+   |                                   ^^^^^^^^^^^^^^ private method
+   |
+  ::: $DIR/auxiliary/other_file.rs:5:5
+   |
+LL |     fn private_method(&self) {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^ private method defined here
+   |