task cannot output absolute symlink to input file

[mlin]

If task creates a symlink to the absolute path of an input file, we reject any use of the symlink in the task outputs, even though it should be OK to output an input file. We could read the link to get the absolute in-container path (which probably doesn't exist on the host!) and determine that it's an allowable output because it's found in TaskContainer.input_path_map_rev.

related: #214