'snap info' should list source-code if existing

[Nathan Teodosio]

https://snapcraft.io/docs/snapcraft-yaml-reference has source-code, "Location where the source of the snap can be found."

'snap info' should show it.

[Maciej Borzecki (maciek-borzecki)]

Note that the `source-code` attribute is part of snapcraft.yaml, not the snap metadata file (meta/snap.yaml). Those attributes are converted into `links[<string>] = []string` in the snap metadata.

Those are shown when you pass --verbose:

```
$ snap info --verbose mesa-2404
...
links:
  contact:
  - https://github.com/canonical/mesa-2404/issues
  source-code:
  - https://github.com/canonical/mesa-2404
  website:
  - https://github.com/canonical/mesa-2404
```

Anyhow, I'll mark this as duplicate of #2064651 which requested to show some of the links by default if present.