Supported aliases are:
- first: The very first version before any migrations have been run.
- current: The current version.
- prev: The version prior to the current version.
- next: The version following the current version.
- latest: The latest available version.
If an existing version number is specified, it is returned verbatimly.
A version number, or null if the specified alias
does not map to an existing version, e.g. if "next"
is passed but the current version is already the
latest.