[#7842] Fix a false positive for Lint/RaiseException

[koic]

Resolve part of #7842.

This PR fixes a false positive for Lint/RaiseException when raising Exception with explicit namespace.

Exception belonging to a namespace is expected to inherit StandardError.

This PR makes Lint/RaiseException aware of the following differences:

Gem::Exception.new.is_a?(StandardError) # => true
Exception.new.is_a?(StandardError)      # => false

On the other hand, the following case have not been resolved by this PR.

module Gem
  def self.foo
    raise Exception
  end
end

Gem.foo #=> Gem::Exception

The above case will be resolved separately from this PR.

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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.