RFE: common package template files - cross-template include

In order to support current Knot DNS packaging which includes a common .service file as well as provide general flexibility and prevent file duplication, apkg should support cross-template file inclusion on package template level.

I propose to do this by introducing include mechanism through a special file INCLUDE_DISTRO_FILES (or better unique name) in individual template directory which contains glob lines relative to distro/ dir, for example:

distro/pkg/deb-custom/INCLUDE_DISTRO_FILES:

pkg/deb/*
common/knot.service

Which would copy matching files along with template files during template rendering. This would allow full reuse of files between individual templates as needed.