WDL tasks that output symlinks to their inputs don't work

[adamnovak]

Julian has a WDL task that wants to output a symlink to an input file (sometimes).

This doesn't work because you get a symlink to the absolute path of the input as mounted in the container, which is a broken symlink when we go to upload it.

When dealing with symlinks coming out of tasks, we should resolve them ourselves according to what we know about what was mounted where in the container filesystem.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1528