Make WDL sibling file imports work even when they come from converting Strings to Files

[adamnovak]

Right now we use some local-to-the-job logic for assigning UUIDs to directories to ensure that sibling files in WDL stay siblings when downloaded elsewhere.

But when converting Strings to Files, two sibling files can be uploaded in different jobs, which means they will get different UUIDs and will not be downloaded as siblings.

We should fix it so they are downloaded as siblings.

We probably don't have to handle the case where the filesystem on the leader changes while a workflow is running, in a way that would matter; we can leave that as undefined behavior.

┆Issue is synchronized with this Jira Story
┆Issue Number: TOIL-1449

[adamnovak]

This is related to #5004 which goes the other way.