Zcu: report multi-module errors against correct file

We were incorrectly using the second referencer as the file to report
against, when it was meant to be the file with the ambiguous module. I
somehow didn't notice this when writing the tests and was explicitly
testing the wrong thing!

src/Zcu/PerThread.zig

@@ -2709,7 +2709,7 @@ fn computeAliveFiles(pt: Zcu.PerThread) Allocator.Error!bool {
                     // have a huge number of them by transitive imports, so just reporting this one
                     // hopefully keeps the error focused.
                     zcu.multi_module_err = .{
-                        .file = file_idx,
+                        .file = res.file,
                         .modules = .{ imported_file.mod.?, imported_mod },
                         .refs = .{ gop.value_ptr.*, imported_ref },
                     };

test/compile_errors.zig

@@ -126,10 +126,10 @@ pub fn addCases(ctx: *Cases, b: *std.Build) !void {
             \\    _ = @import("foo.zig");
             \\}
         , &[_][]const u8{
-            ":1:1: error: file exists in modules 'foo' and 'root'",
-            ":1:1: note: files must belong to only one module",
-            ":1:1: note: file is the root of module 'foo'",
-            ":3:17: note: file is imported here by the root of module 'root'",
+            "foo.zig:1:1: error: file exists in modules 'foo' and 'root'",
+            "foo.zig:1:1: note: files must belong to only one module",
+            "foo.zig:1:1: note: file is the root of module 'foo'",
+            "tmp.zig:3:17: note: file is imported here by the root of module 'root'",
         });
         case.addSourceFile("foo.zig",
             \\const dummy = 0;

test/incremental/change_module

@@ -43,8 +43,8 @@ pub fn main() void {
 comptime {
     _ = @import("other.zig");
 }
-#expect_error=foo.zig:1:1: error: file exists in modules 'root' and 'foo'
-#expect_error=foo.zig:1:1: note: files must belong to only one module
+#expect_error=other.zig:1:1: error: file exists in modules 'root' and 'foo'
+#expect_error=other.zig:1:1: note: files must belong to only one module
 #expect_error=main.zig:3:17: note: file is imported here by the root of module 'root'
 #expect_error=foo.zig:2:17: note: file is imported here by the root of module 'foo'