Bug in adding entries to catalog

When taking an empty catalog:

```
metadata: {}
sources: {}
```

And running:

```
catalog = intake.open_catalog(catalog_uri) # catalog_uri points to empty catalog
source = intake.open_csv(csvfile)
exported_source = source.export(target_uri)
catalog.add(exported_source)
```

The code runs successfully. However, if we have a non-empty calendar such as:

```
metadata: {}
sources:
source1:
args:
meta: {}
urlpath: s3://dummy-bucket/dummy-uri
cache: []
catalog_dir: s3://dummy-bucket/dummy-uri-catalogues
description: ''
direct_access: forbid
driver: intake_parquet.source.ParquetSource
getenv: true
getshell: true
metadata: {}
name: source1
parameters: {}
```

The previous code breaks with `AttributeError: 'LocalCatalogEntry' object has no attribute '_yaml'` from L628 in file `intake/catalog/local.py`. I manage to fix it by changing that line to

`data['sources'][e] = list(entries[e].get()._yaml()['sources'].values())[0]`

But not sure if this will generalize well for all cases.