Add submodule files to gemspec

gem_submodule_files.rb

gem_dir = Pathname.new(__dir__).expand_path
`git submodule --quiet foreach pwd`.split($\).each do |submodule_path|
  Dir.chdir(submodule_path) do
    # for each submodule, cd to foler and get path relative to gem root folder
    submodule_relative_path = Pathname.new(submodule_path).relative_path_from(gem_dir)
    `git ls-files`.split($\).each do |filename|
      # for each git file, prepend relative submodule path and add to spec
      spec.files << submodule_relative_path.join(filename).to_s
    end
  end
end