snap interfaces tab completion does not find snap if it's not using any interfaces

[James Troup]

If I type:

  snap interfaces kube<tab>

I'd expect it to complete to:

  snap interfaces kubectl

However, it doesn't. Presumably because it's only looking at snaps which use an interface. I'd argue it should look at all snaps for completion purposes.

This is with snapd 2.31.1 on Ubuntu 16.04.

[Ian Johnson (anonymouse67)]

The `snap interfaces` command is deprecated, and instead the `snap connections` command should be used. The `snap connections` command does auto-complete in this case. See:

```
$ GO_FLAGS_COMPLETION=1 snap connections go
go
```