fix: update package for Spec in headless repl

[onelson]

Spec was moved during #5116.

This diff updates the import path in the headeless repl to match.

Currently there are no unit tests for this binary, so I had to manually verify the change:

$ go run internal/cmd/headless_repl/main.go                                                                                                                
{"jsonrpc":"2.0","method":"Service.DidOutput","params":[{"input":"now()"}]}                                                                                
{"id":null,"result":{"Result":"2022-08-26T21:07:16.304361483Z"},"error":null}                                                                              
^C^C^C^\SIGQUIT: quit

Checklist

Dear Author 👋, the following checks should be completed (or explicitly dismissed) before merging.

• ✏️ Write a PR description, regardless of triviality, to include the value of this PR
• 🔗 Reference related issues
• 🏃 Test cases are included to exercise the new code (manually verified)
• 🧪 If new packages are being introduced to stdlib, link to Working Group discussion notes and ensure it lands under experimental/ N/A
• 📖 If language features are changing, ensure docs/Spec.md has been updated N/A

Dear Reviewer(s) 👋, you are responsible (among others) for ensuring the completeness and quality of the above before approval.

[onelson]

@Westonside I don't see any tests for the headless repl binary, so I'm not sure how to verify this change. Do you have a recommendation for how to test manually? Maybe you can verify on your side by rebasing on top of this branch?