Use RubyGems to fetch gems

[voxik]

Currently, the gems are fetched via URI.open from custom URL:

gem2rpm/bin/gem2rpm

Lines 52 to 67 in 331c674

	if options[:fetch]
	gem_uri = ''
	begin
	URI("https://rubygems.org/api/v1/gems/#{gemfile}.json").open do |f|
	gem_uri = f.read.match(/"gem_uri":\s*"(.*?)",/m)[1]
	gemfile = URI.parse(gem_uri).path.split('/').last
	gemfile = File.join(out_dir, gemfile)
	open(gemfile, 'w') do |gf|
	gf.write(URI(gem_uri).open.read)
	end
	end
	rescue OpenURI::HTTPError => e
	Gem2Rpm.show_message("Gem fetch failed with error: #{e.message}")
	exit(1)
	end
	end

However, it would be much better if RubyGems are used for this purpose. This would provide more consistent experience, because RubyGems are currently queried for the download URL:

gem2rpm/lib/gem2rpm.rb

Lines 46 to 62 in 331c674

	def self.find_download_url(name, version)
	dep = Gem::Dependency.new(name, "=#{version}")
	fetcher = Gem2Rpm::SpecFetcher.new(Gem::SpecFetcher.fetcher)
	spec_and_source, errors = fetcher.spec_for_dependency(dep, false)
	fail DownloadUrlError, errors.first.error.message unless errors.empty?
	_spec, source = spec_and_source.first
	if source && source.uri
	download_path = source.uri.to_s
	download_path += "gems/"
	end
	download_path
	end