Revert "devel/appstream{,-qt,-compose}: Combine into a single port with SUBPACKAGES."

The current subpackages implementation does not support USES.<subpackage>, which
causes a lot of extra dependencies to be added to the main subpackage.

This reverts commit f292ea6fb5fc10359f130a4e25a3b60ab0df7c6e.

PR: 276584