fix(datastore): do not use a transaction for full pre import repositories
This MR fixes an issue introduced with chore(datastore): add PreImportAll method to Im... (!1236 - merged). We should not run pre imports in a transaction if we're not in a dry run since we're not working with mutable tag data.
Additionally, this MR breaks out the full pre import logic into a separate function to set us up for success with Import Tool: Add One-Shot Three Step Import (#914 - closed)