- Add and improve URL <-> Package URL conversion for gitlab, github,
  cargo, bitbucket and hackage URL conversions
- Add new purl2url conversion utility
- Remove the null=True on Django CharField fields of the PackageURLMixin
- PackageURL.to_dict() now takes an optional "empty" argument with the
  value that empty values to have. It defaults to None which was the
  current behaviour. For some use cases, having an empty string may be
  a better option and this enables this.