Refactor to introduce a `PackageInfo` class

Currently, the `PackageData` class wraps a `package_data` dict as required by setup.py. Various information is then derived by transforming the package names, i.e. the dict keys. It would be cleaner to have a `PackageInfo` structure wrapping all information about a single package and have `PackageData` wrap a list of that. `package_data` can then be generated when needed.