Fix autoload issue with RelativePositioning.range

What does this MR do?

This moves the definition of RelativePositioning.range to lib/gitlab/relative_positioning.rb.

Referencing RelativePositioning::Range would trigger the autoloading of lib/gitlab/relative_positioning/range.rb. But when that class isn't referenced yet, RelativePositioning won't have the range method and we get an exception like:

NoMethodError (undefined method `range' for Gitlab::RelativePositioning:Module):
  lib/gitlab/relative_positioning/mover.rb:33:in `move'
  app/models/concerns/relative_positioning.rb:129:in `move_between'
  app/services/issues/update_service.rb:108:in `handle_move_between_ids'
  lib/gitlab/metrics/instrumentation.rb:160:in `block in handle_move_between_ids'
  lib/gitlab/metrics/method_call.rb:27:in `measure'
  lib/gitlab/metrics/instrumentation.rb:160:in `handle_move_between_ids'
  app/services/issues/update_service.rb:8:in `execute'
  ee/app/services/ee/issues/update_service.rb:23:in `execute'
  app/services/boards/issues/move_service.rb:89:in `update'
  app/services/boards/issues/move_service.rb:60:in `move_single_issue'
  app/services/boards/issues/move_service.rb:10:in `execute'
  app/graphql/mutations/boards/issues/issue_move_list.rb:74:in `move_issue'
  ee/app/graphql/ee/mutations/boards/issues/issue_move_list.rb:21:in `move_issue'
  app/graphql/mutations/boards/issues/issue_move_list.rb:61:in `resolve'

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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