Recreate enums and scalars in mergeSchema

[freiksenet]

Thank you to @orta and @renatobenks for help on this.

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change