Add mixins folder

This moves several files to a new `mixins/` folder. I was going to add
some new macros, per [this comment][1], but thought it might clutter
things up a bit to put them in the root folder. I thought of adding a
`macros/` folder, but decided to emulate the [rubocop source][2]
instead.

[1]: rubocop/rubocop-rspec#934 (comment)
[2]: https://github.com/rubocop-hq/rubocop/tree/master/lib/rubocop/cop/mixin

lib/rubocop-factory_bot.rb

@@ -12,17 +12,19 @@
 require_relative 'rubocop/rspec/wording'
 require_relative 'rubocop/rspec/language'
 require_relative 'rubocop/rspec/language/node_pattern'
-require_relative 'rubocop/rspec/top_level_group'
+
+require_relative 'rubocop/cop/rspec/mixin/top_level_group'
+require_relative 'rubocop/cop/rspec/mixin/variable'
+require_relative 'rubocop/cop/rspec/mixin/final_end_location'
+require_relative 'rubocop/cop/rspec/mixin/empty_line_separation'
+
 require_relative 'rubocop/rspec/concept'
 require_relative 'rubocop/rspec/example_group'
 require_relative 'rubocop/rspec/example'
 require_relative 'rubocop/rspec/hook'
-require_relative 'rubocop/rspec/variable'
 require_relative 'rubocop/cop/rspec/base'
 require_relative 'rubocop/rspec/align_let_brace'
 require_relative 'rubocop/rspec/factory_bot'
-require_relative 'rubocop/rspec/final_end_location'
-require_relative 'rubocop/rspec/empty_line_separation'
 require_relative 'rubocop/rspec/corrector/move_node'
 
 RuboCop::RSpec::Inject.defaults!