Alternative to get_element_value method for HtmlDialog

Yes it appears you haven’t yet learned basic Ruby.

(1) ALL your code needs to be within a top level author / company namespace module.
This means the class and your library module, and your plugins submodules.

(2) To call a module method in another module you need to qualify the method call.

MyNamespace::MyTools::get_dir_list(htmlParam)

… and you must make the module method available from outside by either declaring it as a module method …

module MyNamespace
  module MyTools

    def self.get_dir_list(x_dir)
      puts "Inside get_dir_list #{x_dir}"
      puts listFiles(x_dir)
    end

  end
end

… OR extend the module with itself, creating module method copies of instance methods …

module MyNamespace
  module MyTools

    extend self

    def get_dir_list(x_dir)
      puts "Inside get_dir_list #{x_dir}"
      puts listFiles(x_dir)
    end

  end
end