do not install package again if requested by its virtual package

[Duncaen]

This fix a bug where xbps would install an existing package again if xbps-install is called with one of its provided virtual packages. This change will avoid this, but keep the case where when there is a real package for this virtual package that the real package gets installed.

% xbps-query -p provides strace
cmd:strace-6.15_1
cmd:strace-log-merge-6.15_1
# xbps-install cmd:strace
Name   Action    Version           New version            Download size
strace install   6.15_1            6.15_1                 - 

Size required on disk:        2103KB
Space available on disk:        25GB

Do you want to continue? [Y/n] 

And after the patch:

# xbps-install cmd:strace
ERROR: Package `cmd:strace' already installed.

[classabbyamp]

sgtm