Support Scenario where S3 Path is both a File and a Directory

While running:

```
s3invsync --allow-new-nonempty --ok-errors all s3://linc-brain-mit-staging-us-east-2/linc-brain-mit-staging-us-east-2/staging-configuration /orcd/data/linc/001/s3lincbrain/
```

against `s3invsync --version 0.2.0` the following error occurred:

```
Error: path "zarr/7723d02f-1f71-4553-a7b0-47bda1ae8b42/0/0/0/0" is used as both a file and a directory
```

and confirmed within the AWS console:

This issue captures the need to handle this scenario.

Thanks in advance @jwodder