Merge topic 'fileapi-file-sets-base-dirs-relative'

a3a85524cd fileapi: Fix file sets' base directories relative to top source

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Reviewed-by: Ben Boeckel <ben.boeckel@kitware.com>
Merge-request: !8977