Support Scenario where S3 Path is a File and used to correspond to a Directory and still has version of key there

[yarikoptic]

Originally formulated in

• Support Scenario where S3 Path is both a File and a Directory #170 (comment)

It seems we encountered "design" issue with our solution: it could indeed be legit that we have some old versions (thus would be with versionId's in the suffixes) for keys under key/ subfolder, while the most recent version present in the key file thus forbidding for the key/ to exist at all! e.g. smth like

mkdir -p key; echo content > key/subkey
# snapshot to S3, update inventory
rm -rf key
echo content > key
# snapshot to S3, update inventory