Fix store migration from 1.0 to 2.0 when there is a 'sources' file that would conflict with 'sources' directory #643

Job	Run time
Run pylint that reports only errors	1m 0s
mypy	13s
darker	14s
Diff pylint runs on osc plugin (openSUSE/openSUSE-release-tools)	1m 35s
Diff pylint runs on osc plugin (openSUSE/osc-plugin-collab)	40s
Diff pylint runs on osc plugin (openSUSE/osc-plugin-install)	26s
Diff pylint runs on osc plugin (openSUSE/osc-plugin-overview)	31s
Diff pylint runs on osc plugin (openSUSE/osc-plugin-qam)	55s
Diff pylint runs on osc plugin (openSUSE/sle-prjmgr-tools)	1m 7s
	6m 41s

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