cpan.bbclass: Instead of putting modules into ${libdir}/perl5 put them into

cpan.bbclass: Instead of putting modules into ${libdir}/perl5 put them into
${libdir}/perl5/site_perl/${version} since ${libdir}/perl5 is not part of
the standard INC (search) path for perl and so these modules are not
found by default in the original location. Technically they should go
in vendor_perl instead of site_perl, but that requires changes to all
of the configuration files which is a much more complex change.