Flex: map server-mapped files to the same directory as metadata files

As long as the caller uses a Location to read any files returned by
getUsedFiles(), there should be no difference in behavior.  In OMERO,
this means that all files will be uploaded to the same directory, so
that the server does not need to know about server mappings.

See #11781.
Merge pull request #819 from melissalinkert/flex-used-files

Flex: map server-mapped files to the same directory as metadata files