scripts/package-metadata.pl: honour DEFAULT_VARIANT

So far, package-metadata.pl always considered the first provider of a virtual
package to be the default variant which might deviate from what buildroot
considers to be the default.

Change the Kconfig dependency / select code generation for virtual package
providers to consider the DEFAULT_VARIANT to be the primary provider and only
fall back to the first provider if no default variant was explicitely tagged.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>