Fix store migration from 1.0 to 2.0 when there is a 'sources' file that would conflict with 'sources' directory #607

Job	Run time
unit - read only fixtures	25s
unit (registry.fedoraproject.org/fedora:38)	1m 6s
unit (registry.fedoraproject.org/fedora:39)	53s
unit (registry.fedoraproject.org/fedora:rawhide)	51s
unit (registry.opensuse.org/opensuse/leap:15.3)	1m 31s
unit (registry.opensuse.org/opensuse/leap:15.4)	1m 35s
unit (registry.opensuse.org/opensuse/leap:15.5)	1m 20s
unit (registry.opensuse.org/opensuse/tumbleweed)	1m 6s
unit (quay.io/centos/centos:stream9)	32s
unit (docker.io/library/debian:stable)	27s
unit (docker.io/library/debian:unstable)	37s
unit (docker.io/library/ubuntu:latest)	32s
behave	6m 4s
	16m 59s

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