Allow collection to be on file system while catalog is in the cloud

When using an esm_collection from file system:

`intake.open_esm_datastore('/home/dkrz/k204210/intake-esm/esm-collections/dkrz_era5_disk_grb_fromcloud.json')`

with url as `catalog_file` entry value:

```
cat /home/dkrz/k204210/intake-esm/esm-collections/dkrz_era5_disk_grb_fromcloud.json
...
"catalog_file": "https://swift.dkrz.de/v1/dkrz_a44962e3ba914c309a7421573a6949a6/intake-esm/dkrz_era5_disk_grb.csv.gz",
```
results in
```
Unable to find: /home/dkrz/k204210/intake-esm/esm-collections/https:/swift.dkrz.de/v1/dkrz_a44962e3ba914c309a7421573a6949a6/intake-esm/dkrz_era5_disk_grb.csv.gz
```

I think there is no reason why this should not work.

Start by reproducing the issue with intake.open_esm_datastore and the local JSON file whose catalog_file points to the Swift URL. Trace how catalog_file is resolved when opening an esm_collection; done means a local collection can load its remote catalog without prepending the local collection directory.

Written by the indexing model from the issue text.