RuboCop: Split `.rubocop_manual_todo.yml` into `.rubocop_todo/**/*.yml`

What does this MR do and why?

1. RuboCop: Split .rubocop_manual_todo.yml into .rubocop_todo/**/*.yml
2. Enhance rake rubocop:todo:generate to split TODOs into separate files

This MR moves all RuboCop configuration from the large .rubocop_manual_todo.yml into smaller more focused files within .rubocop_todo/**/*.yml.

This step enables us to split .rubocop_todo.yml later which is very likely needed if we start using rake rubocop:todo:generate because it will produce a huge .rubocop_todo.yml (due to much higher "exclude limit").

Contributes to #294273 (closed).

Verification of rubocop --show-cops

The output of bundle exec rubocop --show-cops did not change before and after this MR (with and without revealing todos) which was expected:

Before

$ export REVEAL_RUBOCOP_TODO=0
$ bundle exec rubocop --show-cops | md5sum
dbf75c20c512b4c25f263650c1e205cd  -

$ export REVEAL_RUBOCOP_TODO=1
$ bundle exec rubocop --show-cops | md5sum
6d1a0f5e6119f7440839fc65fbd823c8  -

After

$ export REVEAL_RUBOCOP_TODO=0
$ bundle exec rubocop --show-cops | md5sum
dbf75c20c512b4c25f263650c1e205cd  -

$ export REVEAL_RUBOCOP_TODO=1
$ bundle exec rubocop --show-cops | md5sum
6d1a0f5e6119f7440839fc65fbd823c8  -

How to set up and validate locally

export REVEAL_RUBOCOP_TODO=1
bundle exec rubocop --show-cops

export REVEAL_RUBOCOP_TODO=0
bundle exec rubocop --show-cops

bundle exec rubocop <file>

Making of

The YAML files have been generated via the following script:

require "fileutils"
require "yaml"
require "active_support/all"

ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym 'RSpec'
  inflect.acronym 'GraphQL'
end

def dump_into_file(cop_name, config)
  filename = File.expand_path(File.join(".rubocop_todo/#{cop_name.underscore}.yml"), __dir__)

  FileUtils.mkdir_p(File.dirname(filename))

  File.write(filename, YAML.dump(cop_name => config))
end

yaml = YAML.load_file(".rubocop_manual_todo.yml")
yaml.each do |cop_name, config|
  dump_into_file(cop_name, config)
end

Later, rake rubocop:generate:todo could use the very same technique to split the auto-generated .rubocop_todo.yml into .rubocop_todo/**/*.yml too. Note, with this is approach we are losing YAML comments generated by RuboCop but I think this is acceptable.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.