Gem::Commands::ContentsCommand#files_in_gem

# File tmp/rubies/ruby-3.4.1/lib/rubygems/commands/contents_command.rb, line 93
def files_in_gem(spec)
  gem_path  = spec.full_gem_path
  extra     = "/{#{spec.require_paths.join ","}}" if options[:lib_only]
  glob      = "#{gem_path}#{extra}/**/*"
  prefix_re = %r{#{Regexp.escape(gem_path)}/}

  Dir[glob].map do |file|
    [gem_path, file.sub(prefix_re, "")]
  end
end