Alternatives to `port installed`.

This PR is not for merging, it's to discuss how RVM detects installed ports. I've presented to alternatives, discussed below.

`port installed` can be slow, running a complete list for each library check increases run time.  Asking for a specific port name is faster, but is it suitable?

Also, the check does not consider 'inactive' ports.  For example, with `openssl` installed but inactive, this will result in a false positive for a satisfied dependency.  I think we should check against the list of active ports.