Support for virtual packages

XBPS lacks support for virtual packages. This kind of packages provide the same functionality and therefore any of them can be installed. Such example would be 'system-log-daemon' that rsyslog, sysklogd, syslog-ng and others provide.

Proposed implementation is to provide the /etc/xbps.plist conf file with the following structure:

<key>virtual-packages</key>
<dict>
  <key>system-log-daemon</key>
  <array>
    <string>rsyslog</string>
    <string>syslog-ng</string>
  </array>
</dict>