IngestExternalFile put file on bottom level but on db_paths[0]

### Expected behavior
`IngestExternalFile` should put file on db_paths according to level.

### Actual behavior
`IngestExternalFile` put file on bottom level but on `db_paths[0]`.

### Steps to reproduce the behavior
1. Set `Options` with level style compaction
2. Set `Options` with multiple `db_paths`
3. Open db with defined Options
4. IngestExternalFile

Start by reproducing the issue with level-style compaction and multiple db_paths, following the IngestExternalFile path-placement logic. Check how the target level is selected and why placement uses db_paths[0]. Done when ingested files are placed on the db_path corresponding to their level, with regression coverage for this configuration.

Written by the indexing model from the issue text.