Refactor back file system listing `source` and `path`

[ilongin]

When trying to get rid of add-storage method we refactored FS listing to always set root of the file system (file:///) as source and rest of the path is in path column. Now that we moved listing to application level, we can be without add-storage without changing source and path which means they should be returned to the original implementation as it was before.

When someone does: DataStorage.from_storage("file:///home/ivan/datasetsa/animals") we should set file:///home/ivan/datasetsa/animals as source and other remaining part of the full path of specific file will go to path.

This is important as current solution could create problems for users as it is different from other source types / clouds like s3 or `azure', but instead they should all be consistent.