virtualbox: Split kernel modules into own package

Putting the kernel modules into the same output path as the main
VirtualBox derivation causes all of VirtualBox to be rebuilt on every
single kernel update.

The build process of VirtualBox already outputs the kernel module source
along with the generated files for the configuration of the main
VirtualBox package. We put this into a different output called "modsrc"
which we re-use from linuxPackages.virtualbox, which is now only
containing the resulting kernel modules without the main user space
implementation.

This not only has the advantage of decluttering the Nix expression for
the user space portions but also gets rid of the need to nuke references
and the need to patch out "depmod -a".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>