add ability to get a absolute file path when possible

[wjwwood]

In the cases where file://... or package://... are the input, it would be nice to get an absolute path to the resource on the filesystem rather than the data. This is obviously not possible if the source is off-disk (like http:// or the proposed param:// in #12), but when it's possible the feature would have been used here:

ros2/rviz@1a3b84f#r177022950