Support URL and path dependencies without a package name

Apparently this works with pip?

```
git+https://github.com/pallets/flask.git@refs/pull/5313/head
```

As opposed to:
```
flask @ git+https://github.com/pallets/flask.git@refs/pull/5313/head
```

We could decide not to support these. I don't know if they're spec-compliant.
