The fix for GH#709 updated FindJSONLInDir to skip interactions.jsonl,
but the doctor check in CheckLegacyJSONLFilename was not updated.
This caused `bd doctor` to warn about multiple JSONL files when
both issues.jsonl and interactions.jsonl exist (which is normal
after `bd init`).
Also skip molecules.jsonl which is another system file that shouldn't
be counted as a duplicate issue database.