stac copy pulls in external catalogs/collections

[constantinius]

Describe the bug I have a local catalog referencing a remote catalog/collection. When I copy my local collection to some place using stac copy, the remote collection is saved right next to the local one.

To reproduce

catalog.json contents:

{
    "id": "test",
    "type": "Catalog",
    "stac_extensions": [],
    "stac_version": "1.0.0",
    "description": "test",
    "title": "test",
    "links": [
      {
        "rel": "child",
        "href": "https://raw.githubusercontent.com/radiantearth/stac-spec/v1.0.0/examples/collection.json",
        "type": "application/json"
      }
    ]
  }

1. Install stactools
2. Run stac copy catalog.json out
3. Run tree

.
├── out
│   ├── catalog.json
│   └── simple-collection
│       ├── 20201211_223832_CS2
│       │   └── 20201211_223832_CS2.json
│       └── collection.json
└── test.json

Expected behavior The remote collections should not be copied locally, at least there should be a way to opt out of that behavior.

Screenshots and shell session dumps

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Additional context

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