[INTERNAL] Let FileListFactory use Saros IResources

This made two types of conversions in calling classes unnecessary:
Between Eclipse IResources and Saros IResources, and from (Eclipse)
CoreException to (java.io) IOException.
The corresponding code snippets were simplified to improve readability.

Change-Id: Ib821a7a213493e7b518ab312b2d1a3fd8a903e15
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1563
Tested-by: Jenkins CI
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>