Flutter Build Runner - Generate all files to a single folder

Question

I have this current build runner setup which generates all the files under lib/gen.

targets:
  $default:
    builders:
      dart_mappable_builder:
        options:
          build_extensions:
            'lib/{{file}}.dart': 'lib/gen/{{file}}.mapper.dart'

This is the generated structure

lib
 - gen
  - folder1
    ** files
  - folder2
    ** files

What I am trying to achieve

lib
 - gen
  ** files

How do I generate all the files in one folder without the subdirectories?

Thank you.