Reject nonregular configuration files before opening them

imports.go

@@ -382,6 +382,18 @@ func (s *importTraversal) readImport(relative, sourceID string, optional bool) (
 	if limit < 0 {
 		limit = 0
 	}
+	preflight, err := s.root.Stat(relative)
+	if err != nil {
+		if optional && errors.Is(err, fs.ErrNotExist) {
+			return nil, true, nil
+		}
+		value := classifySourceError(err, sourceID)
+		return nil, false, &value
+	}
+	if !preflight.Mode().IsRegular() {
+		value := sourceDiagnostic(CodeSourceNotRegular, sourceID, "source is not a regular file")
+		return nil, false, &value
+	}
 	file, err := s.root.Open(relative)
 	if err != nil {
 		if optional && errors.Is(err, fs.ErrNotExist) {