a.rb

require_relative 'import'

B = import "b"

p defined?(::User)

user = B.new_user
p user

b.rb

class User
end

def new_user
  User.new
end

import.rb

class ModuleLoader
  def self.instance
    @instance ||= new
  end

  def initialize
    @modules = {}
  end

  def import(path)
    @modules[path] ||= load(path)
  end

  def load(path)
    mod = @modules[path] = Module.new
    code = File.read(path)
    mod.class_eval("module_function\n#{code}", path, 0)
    mod
  end
end

module Kernel
  def import(name)
    file = "#{name}.rb"
    ModuleLoader.instance.import(file)
  end
end