Allow copyright/license in subpackages

[bgermann]

While the APKBUILD allows one to set the license for subpackages (see e.g. APKBUILD for util-linux), melange does not allow that. Please make it possible. This is very interesting for packages that have distinct sets of software that might have incompatible licenses. Also see util-linux for that, which has "GPL-3.0-or-later (...) AND GPL-2.0-only" in its license list (which are incompatible but applicable to distinct software sets).

[xnox]

Indeed ditto for glibc and gcc.... where runtime libraries license is widely different from developer-header license.