File src/core.js has started erroneously being copied to dist/ in jquerygh-2981.

Fixes jquerygh-4489
Closes jquerygh-4492
Ref jquerygh-2979
Ref jquerygh-2981

Summary

Checklist

• All authors have signed the CLA at https://cla.js.foundation/jquery/jquery
• New tests have been added to show the fix or feature works
• Grunt build and unit tests pass locally with these changes
• If needed, a docs issue/PR was created at https://github.com/jquery/api.jquery.com