revert: gracefully handle missing manifest revisions during imports

This MR reverts the fix from !977 (merged), while keeping the added tests.

We're reverting the fix because we've recently discovered what seems to be a widespread issue related to buildkit images having references with unknown manifest revisions (#651 (closed)). The fix has been released already but has not yet been used on .com as the migration is currently blocked due to #652 (closed).

We'll need to find out what is going on with #651 (closed) before we can confidently bring back this fix.