Fix resolving mixins that mix themselves in

[ccutrer]

Description

module A
  B.include(self)
end

Wasn't working properly

Completed Tasks

• I have read the [Contributing Guide][contrib].
• The pull request is complete (implemented / written).
• Git commits have been cleaned up (squash WIP / revert commits).
• I wrote tests and ran bundle exec rake locally (if code is attached to PR).

[lsegal]

Sorry about the delay-- thanks for the PR!