Instead of assigning various variables ourself, rely on the common logic
provided by "packaging framework". This brings this package in sync with
other ones which also use git repos as source, dates and commit ids as
version etc. This results also in using xz for tarballs. While at,
populate the PKG_MIRROR_HASH.

Thanks @diizzyy for suggesting these improvements.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>