refactor(datastore): use safe CreateOrFindByPath in importer

Hayley Swimelarrequested to merge
importer-safe-find-and-create into master

With !795 (merged) and !796 (merged) it is now safe to call CreateOrFindByPath without incrementing primary keys for repositories and top-level namespaces. This MR refactors the importer to call this function, instead of FindByPath then CreateByPath seperately.

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