buildsys: improve resolving package meta info

buildsys: improve resolving package meta info

use the full version for the cache invalidation technique and fall
back to the commit ID and current time.

With that we can drop the exports in the top-level makefile, which
would only work for local direct builds anyway, but not for building
from DSC source package.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>