repoman: Warn if virtuals depend on perl-core, bug 516428

Reported-by:   Andreas K. Hüttel  <dilfridge@gentoo.org>
Authored-by:   Andreas K. Hüttel  <dilfridge@gentoo.org>
Reviewed-by:   Alexander Berntsen <bernalex@gentoo.org>
Signed-off-by: Alexander Berntsen <bernalex@gentoo.org>