Fix explicit project import to relative

lib/models/credentials.dart

@@ -1,6 +1,7 @@
-import 'package:flutter_portal/models/exec_error.dart';
 import 'package:result_monad/result_monad.dart';
 
+import 'exec_error.dart';
+
 class Credentials {
   final String username;
   final String password;