How to detect filename conflicts with uninstalled ports?

[Charlie Kester]

I'm working on a new port that installs a program with a name that seems
like it would already be taken, but I don't have anything by that name
on my system -- even though I have lots of ports installed.

I know that I can use "portsdb -r" to find all the ports that depend on
a given port, whether they're installed or not.  Is there a similar way
to determine if an uninstalled port installs a file with a given name?