Improvements to unused helper methods linter

The following discussions from !139028 (merged) should be addressed:

• @splattael started a discussion: (+2 comments)

  Question Do we need handle method name duplicates? 🤔

  p helpers.map { |x| x[:method] }.size      # => 2463
  p helpers.map { |x| x[:method] }.uniq.size # => 2285

• @splattael started a discussion: (+1 comment)

  Question Do we care about lib? 🤔

  I see some use of method helpers in lib/api, for example: repository_mirror_attributes:

  • attrs = attrs.except(*::EE::ApplicationSettingsHelper.repository_mirror_attributes)
  • expose(*EE::ApplicationSettingsHelper.repository_mirror_attributes, if: ->(_instance, _options) do

  See also #330633 (comment 573421455).

• @splattael started a discussion: (+2 comments)

  Suggestion Could we use filter_map instead? 🤔

  unused = Parallel.filter_map(helpers, progress: 'Checking helpers') do |helper|
    helper unless /(?<!def )#{helper[:method]}/.match?(source_code)