/usr/lib/ruby/2.3.0/rubygems/resolver/ is in libruby2.3 2.3.0-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/2.3.0/rubygems/resolver/activation_request.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/api_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/api_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/best_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/composed_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/conflict.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/current_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/dependency_request.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/git_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/git_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/index_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/index_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/installed_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/installer_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/local_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/lock_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/lock_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/dependency_graph.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/errors.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/gem_metadata.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/modules/
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/modules/specification_provider.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/modules/ui.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/resolution.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/resolver.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/molinillo/lib/molinillo/state.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/requirement_list.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/spec_specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/specification.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/stats.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/vendor_set.rb
/usr/lib/ruby/2.3.0/rubygems/resolver/vendor_specification.rb