Consider host-specific all source sets that participate in a compilation
of a target that cannot be built on some of the hosts.
Exclude those source sets from the `-metadata` artifact since the
`metadata` target must publish in the same way from any host.
Instead, add a metadata variant to those targets which use such a source
set and pack the source set's klib into a similar metadata JAR as the
artifact of those metadata variants.
Sergey Igushkin
f30aada84a