ls without -r on a folder (local or remote) should list only immediate assets and folders

ATM regardless with `-r` or not lists all the assets under it:

```
(git)lena:~/proj/dandi/dandi-cli-master[rf-organize]git
$> dandi ls -f pyout https://gui.dandiarchive.org/#/dandiset/000026/draft/files?location=derivatives%2FEPIC%2F
ASSET_ID PATH SIZE CREATED MODIFIED
0fd21831-d3cd-462a-a044-a164472e5c5c derivatives/EPIC/dataset_description.json 1.1 kB 2021-04-21T08:45:29.941149Z 2021-04-21T08:45:29.941176Z
7f676530-ad89-4f55-9d9e-c8597458cba9 derivatives/EPIC/sub-I46/sub-I46_ses-MRI_flip-1_VFA.json 269 Bytes 2021-04-21T08:45:29.955628Z 2021-04-21T08:45:29.955665Z
bf18ba02-ac02-4860-a5cc-02395f1b8cae derivatives/EPIC/sub-I46/sub-I46_ses-MRI_flip-2_VFA.json 269 Bytes 2021-04-21T08:45:30.039613Z 2021-04-21T08:45:30.039636Z
60f34d8f-4185-478f-9f33-95be79815b8a derivatives/EPIC/sub-I48/sub-I48_ses-MRI_flip-1_VFA.json 269 Bytes 2021-04-21T08:45:31.242278Z 2021-04-21T08:45:31.242302Z
623b6186-3f36-4796-99b4-0f13764dd74a derivatives/EPIC/sub-I46/sub-I46_ses-MRI_flip-3_VFA.json 269 Bytes 2021-04-21T08:45:31.473047Z 2021-04-21T08:45:31.473129Z
9aaec851-f443-4b71-bea9-eebceadabd82 derivatives/EPIC/sub-I48/sub-I48_ses-MRI_flip-2_VFA.json 269 Bytes 2021-04-21T08:45:33.160120Z 2021-04-21T08:45:33.160179Z
cac05891-3282-4141-ad01-e1366c47aa85 derivatives/EPIC/sub-I46/sub-I46_ses-MRI_flip-1_VFA.nii.gz 2.6 GB 2021-04-21T08:46:24.165330Z 2021-04-21T08:46:24.165354Z
82a2e531-7c78-4e17-92d0-e818b38c2e80 derivatives/EPIC/sub-I48/sub-I48_ses-MRI_flip-3_VFA.json 269 Bytes 2021-04-21T08:46:26.226670Z 2021-04-21T08:46:26.226693Z
22135e74-35f5-4cd3-903b-91dc898064d4 derivatives/EPIC/sub-I46/sub-I46_ses-MRI_flip-2_VFA.nii.gz 2.6 GB 2021-04-21T08:46:26.513638Z 2021-04-21T08:46:26.513661Z
2584be56-e203-4290-91b0-d7bc7b1c81e9 derivatives/EPIC/sub-I46/sub-I46_ses-MRI_flip-3_VFA.nii.gz 2.6 GB 2021-04-21T08:46:27.389322Z 2021-04-21T08:46:27.389346Z
519e3aa9-e2e4-4344-9d9c-b01b5f681989 derivatives/EPIC/sub-I48/sub-I48_ses-MRI_flip-4_VFA.json 269 Bytes 2021-04-21T08:46:29.222711Z 2021-04-21T08:46:29.222769Z
...
```
support for that is conditioned on the destiny of the extended mode for `/files` endpoint: https://github.com/dandi/dandi-api/pull/261