Gracefully handle missing tags prefix during imports

Context

Found in &7528 (comment 892958210).

Problem

The importer is not handling repositories with no tags folder gracefully. Imports are failing with error pre importing tagged manifests: reading tags: unknown repository name=....

Solution

Handle missing tags folder gracefully. When there is no tags folder there are no tags, when there are no tags there is nothing to import, therefore we should just complete the (pre)import and flag the repository as successfully imported.

Note that this only applies to pre-imports. These always come before the corresponding import, so if the tags prefix went missing between the pre and final import, we should let that be visible as an error as it's not expected at all. This is wrong. We need to do it for final imports as well, as we'll list the tags folder there as well, so the same exception will occur.