checking if a port is installed (even if not active) from a PortGroup?

[René J . V . Bertin]

Hi,

We can check for active ports, but what about any installed ports? Does any of 
the registry functions exported in the macports packages allow this or if not, 
is it possible to `require` one ore more packages to obtain access to the 
`registry::installed` procedure?

Thanks,
R.