Macos-pkg-builder

- Fix signing issue with `pkg_signing_identity` parameter

- Add support for pkg signing
- Configured via new `pkg_signing_identity` parameter

- Add support for bundling additional resources for pre/postinstall scripts to access
- Files to be defined via new `pkg_script_resources` parameter
- Current working directory for scripts will be the same directory containing additional resources

- Create output directory if it doesn't exist

- Resolve import errors from PyPi

- Initial release