HPCC-8521 Fix file cloning while copying queries not resolving indexes

Look for Node/Att/@name="_indexFileName" as well as "_fileName"
in the workunit graphs when gathering file references.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>
Merge pull request #3822 from afishbeck/resolve_index_files8521

HPCC-8521 Fix file cloning while copying queries not resolving indexes

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>