pkg / db: added subpkgver support

Adds support for abuild subpkgver feature for abuild!137

Please note that I am not an experienced lua developper. I'm sure there are better ways to do certain things, so don't hesitate to comment.

Possible improvements:

• argument parsing could be more robust. For example, it is always assumed that version information will be the last argument in subpackages array. The version information detection logics could also be more robust, relying on checking if argument has a dot to see if it contains version information. This shouldn't be a problem as subpkgname, subpkgsplit and subpkgarch should not have that character, thus making them mutually exclusive.

Accompanies abuild!137 Fixes part of: abuild#10060