download: RF for most logical handling of `--output-dir` and (specified) dandiset

[yarikoptic]

ATM we download into --output-dir/DANDISET_ID/ if URL points to the dandiset or just --output-dir/PATH if PATH points to the folder or file. Kinda simple/sensible -- output-dir points to a directory to download specified entity under.
But as we would introduce download of versioned dandisets, such behavior might be suboptimal. What if I want to download a specific version of the dandiset under a specifically named directory (e.g. 000001-v1) and draft under plain 000001. download of cause could just (always?) append version of the dandiset into the dandiset top level directory name, or it could become an option and then we just keep current behavior.
Minor, but something to decided on