template: support INCLUDE_DISTRO_FILES

Files named INLCUDE_DISTRO_FILES can now be inserted into packaging templates which contain one glob per line relative do distro dir. All files and directories matching the glob are rendered during templating.

This can be used to include common files for example from distro/common, from other templates like distro/pkg/deb or anywhere else in distro/.

apkg doesn't have a use for this yet so testing is postponed for Knot DNS integration soon which will properly test this feature.

Fixes: #10 (closed)