Hierarchical relative target path determination.
Determining an artifact's target path from its source path is currently done in a wholesale manner by SourcePathMummifier.getArtifactTargetPath(), the base version of which merely swaps out the site source root path with the site target root path. This does not allow for subtleties of change inside the path, in a path segment a mummifier might control.
For example a directory mummifier might consider the _ in its name to represent a veiled artifact. The directory mummifier would unveil the target path for the artifact (), and mummifiers for child artifacts should not have to worry about this.
In other words, a mummifier should determine the next target path one path segment at a time at each level rather than redetermining the path from the root each time. This allows each mummifier to control the path format of its level.