core/pkg-virtual: fix fallout of host-dependecy no auto-derivation

core/pkg-virtual: fix fallout of host-dependecy no auto-derivation

In 4bdb067 (infra: remove auto derivation of host dependencies), the
dependencies of host packages are no longer inherited from the
dependencies of the corresponding target package.

However, for virtual packages, there is no provider declared for the
host variant, so we end up with no dependency for the host variant
of virtual packages.

This causes host-luainterpreter to have no provider, and thus breaks the
build of host-luarocks.

To fix that, we use the host variant of the provider of the target
variant of the virtual package to define the provider of the host variant
of the virtual package (re-read it, it's correct!).

Fixes:
    http://autobuild.buildroot.org/results/d8b/d8baf44b1e2f8e0e32c86558fc3e0694235b512a
    http://autobuild.buildroot.org/results/59a/59ac0fe84e5c76a590c698a1483b2a3ae26e1dcf
    http://autobuild.buildroot.org/results/203/2039f24731d8e14cde24c613b487b3db530dd238

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>