tzdb_initialize() is intended to be called from a client package's
.onLoad() as tzdb::tzdb_initialize() to ensure that the tzdb package
has been loaded.
The function itself doesn't do anything. It is instead called for the
side-effect of loading the tzdb package. This does two things:
The tzdb .onLoad() hook is run, which sets the path to the time zone
database.
The callables in tzdb are registered, which allows them to be called from
other packages.