[BUG]: `target_root` usually ignored for `auto` and `auto-existing`

[mtalexan]

On line 836 the post-transformed target path is prepended with the target_root only if the target isn't already absolute.

There doesn't seem to be any valid case where the path_replacements or fs_path_replacements destinations shouldn't be absolute though. The only case it seems to make a difference is here, when the target is being calculated for a specific file, and at that point there's no valid use case where target_root shouldn't be prepended to the result.