Code duplication: datachain.lib.listing should use Client.scandir()

[rlamy]

datachain.lib.listing contains a partial copy of the code used by Client.scandir(), which only includes a generic version of the "nested" algorithm. This has some disadvantages:

• Code duplication is always bad
• It misses customisations added for some of the clients
• Using the "nested" algorithm will cause a regression on https://github.com/iterative/dvcx/issues/1403 as soon as we start using it in Refactor DataChain.from_storage() to use new listing generator #294.

A quick fix is to convert list_bucket() into a wrapper around Client.scandir(), converting the Entry objects the latter returns into File, but it shouldn't be hard to make Client.scandir() return File directly, as already planned in #33.