Split plugins into separate sub-packages
    
    Since annobin is required by redhat-rpm-config, having the clang and llvm plugins
    as part of the default package means that llvm-libs is always installed into
    the buildroot, which is unnecessary for most packages.
    
    Also, having an llvm depenency in the buildroot makes upgrading llvm more difficult.
    
    This patch attempts to resolve these issues by spliting up the various plugins
    into their own sub-packages, so that redhat-rpm-config can have finer-grained
    dependencies on only the plugins that it needs.