Allow configuring other paths with respect to state_dir.

If you override state_dir, you might think that you've done enough to avoid using the default state directory in ${ARTI_LOCAL_DATA} ... but you'd be wrong! The keystore location also defaults to ${ARTI_LOCAL_DATA}/keystore, so you'd need to override that too.

It would be neat if we could define a directory as ${state_dir}/keystore. But in order to do that, we would need to have the ability to let some CfgPath-like types get defined with respect to other CfgPaths. We would need some kind of a multi-step path resolution process.

To keep this simple, I'd suggest that we declare that there is only one level of dependencies here: that other paths may be defined in terms of ${state_dir}, but that we don't allow every possible path to depend on every other possible path.

Is this worth doing?