Fix a false positive for Style/ClassAndModuleChildren

[koic]

This PR fixes the following incorrect auto-correct for Style/ClassAndModuleChildren when using cbase module name.

% cat example.rb
module ::Fixtures
end

% bundle exec rubocop -A --only Style/ClassAndModuleChildren example.rb
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:8: C: [Corrected] Style/ClassAndModuleChildren: Use nested
module/class definitions instead of compact style.
module ::Fixtures
       ^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected

% cat example.rb
module
  module Fixtures
  end
end

% ruby -c example.rb
example.rb:3: syntax error, unexpected '\n', expecting '.' or &. or ::
or '['

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.