Correctly build subpackage dependencies without installing them to build the package

A previous attempt inserted subpackage depends into the package depends list returned by get_depends().

This meant that pmb would try to install subpackage depends when building the package, which breaks for some cases like device packages with kernel variants since the depends of different subpackages might conflict with each other.

Let's instead just add the subpackage depends to the build queue, so they get built like package depends, without being pulled in as build dependencies for the package.

Fixes #2415 (closed)