Added support for system/config virtualpkg.d directories.

The system virtualpkg directory set to <rootdir>/usr/share/xbps/virtualpkg.d contains
virtualpkg configuration files (.conf/.vpkg) that can be overrided by the admin
in <rootdir>/etc/xbps/virtualpkg.d bearing the same file name.

This obsoletes the "virtualpkgdir" keyword support from the xbps configuration file.