action_info: eliminate duplicate info_pkgs match

action_info: eliminate duplicate info_pkgs match

Currently, sys-kernel/linux-headers is matched by both a plain
sys-kernel/linux-headers atom and by the virtual/os-headers new-style
virtual. For backward compatibility, we're going to have duplicates
like this for at least a few months (see bug #364673, comment #5).
Therefore, automatically eliminate duplicates in the display. Entries
that include virtual provider info are preferred over those that do
not.