Allow `*` as third-party version

[srittau]

We have a few data-only packages (most recently tzdata) or calver packages where the API rarely changes (regex). I suggest we allow * as version for third-party packages. This will always use the full latest upstream version as part of the uploaded stubs. In the future, we could also extend the stub uploader to automatically upload a new stub version when a new upstream package is released.

This would make the version field match closer to what upstream version is actually supported and would reduce maintenance burden for maintainers.