main.rb

require "proxy"

# get the name of the manager
# let's say it's pacman in our case

pacman = PkgManagerProxy.new("pacman")
pacman.install("tmux")

pacman.rb

class Pacman
  def install(*args)
    puts "Installing: #{args.inspect}"
  end
end

proxy.rb

class PkgManagerProxy
  def initialize(name)
    @instance = create_instance(name)
  end

  def method_missing(name, *args, &block)
    @instance.public_send(name, *args, &block)
  end

  def respond_to_missing?(*args)
    @instance.respond_to?(*args)
  end

  private

  def load_implementation(name)
    # determine the path
    require_relative "#{name}.rb"
  end

  def create_instance(name)
    load_implementation(name)

    Object.const_get(name.capitalize).new
  end
end