docs: def build_ingest_fs(self):
          """Return a pyfilesystem subdirectory for the ingested source files"""
  
          base_path = 'ingest'
  
          if not self.build_fs.exists(base_path):
              self.build_fs.makedir(base_path, recursive=True, allow_recreate=True)
  
          return self.build_fs.opendir(base_path)