version: Remove build identifier parsing

The SysrootVersion class was trying to define a date and build index
from the build identifier. While this is true by convention for the
Apertis builds, this may break in the future.

Instead treat the build identifier as an opaque string consisting of just
digits and dots.

While there, also make all SysrootVersion arguments optional to improve
readability and rename author to variant (which is its intended
meaning).

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Differential Revision: https://phabricator.apertis.org/D6263