batch dependencies

[v4hn]

Some modules like lightdm, thunar-thumbnailer, etc. have options which
add a dependency on more than one other module.
The only "proper" way we have to handle this at the moment is to add a PROFILE module which depends on all these dependencies and then add an optional_dependency for that PROFILE. For obvious reasons this was never done.

We need a way to specify such batch dependencies in DEPENDS
without hacking around the framework via "(select together with XYZ)" descriptions.