Add support for buildpack.toml manifest

This is a package manifest format for Heroku and Packeto:
See https://github.com/search?q=path%3A**%2Fbuildpack.toml&type=code&ref=advsearch

Adding support would consist in:

1. Research and document here as comment the buildpack format specification and various spec versions
2. Collect a few representative test files and attach as comment keeping track of the URL provenance for each
3. Create a new packagedcode/buildpack.py module with an overall structure similar to that of existing modules. Implement parsing and creating a Package.
4. Add tests in a new test_buildpack.py test module using the collect test files
5. Add extra unit tests for single functions