Make sure we're looking at the right package_file

If we create a new package, the factory will also create a corresponding
package file, which will share the same name as the package file we're
creating explicitly, and it only has file_sha1 defined therefore if
PostgreSQL happens to load the package file sharing the same name from
the package, then it will fail.

This make sure we don't create a new package thus avoid having an extra
package file which will conflict with the package file we're testing
here.

spec/services/packages/rubygems/create_gemspec_service_spec.rb

@@ -4,10 +4,10 @@ require 'spec_helper'
 RSpec.describe Packages::Rubygems::CreateGemspecService do
   include RubygemsHelpers
 
-  let_it_be(:package) { create(:rubygems_package) }
   let_it_be(:package_file) { create(:package_file, :gem) }
   let_it_be(:gem) { gem_from_file(package_file.file) }
   let_it_be(:gemspec) { gem.spec }
+  let_it_be(:package) { package_file.package }
 
   let(:service) { described_class.new(package, gemspec) }