Fix regression in enum input -> internal value conversion

[tgriesser]

Fixes the inverse of #973, values on input should be mapped to their internal values. This was broken because of the changes noted here. I took what this was trying to achieve and moved it into a separate transform specific to merged schema delegation. Tests included.

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. Include a description of your change, link to PR (always) and issue (if applicable). Add your CHANGELOG entry under vNEXT. Do not create a new version number for your change yourself.

[hwillson]

Awesome @tgriesser - thanks very much!

[hwillson]

Available in graphql-tools 4.0.2. Thanks!