Use vendored shared functions

## Description of proposed changes Replace custom `resolve_path` with the vendored function from https://github.com/nextstrain/shared. ## Related issue(s) Related to https://github.com/nextstrain/pathogen-repo-guide/issues/83 ## Checklist <!-- Make sure checks are successful at the bottom of the PR. If applicable, add: - any changes to existing tests - any additional manual testing to confirm changes Please add a note if you need help with adding tests. --> - [x] Checks pass <!-- 🙌 Thank you for contributing to Nextstrain! ✨ -->