This adds options to `spack list` that allow you to list only packages from specific
repositories/namespaces, e.g.:
```console
spack list -r builtin
```
only lists packages from the `builtin` repo, while:
```console
spack list -r myrepo -r myrepo2
```
would list packages from `myrepo` and `myrepo2`, but not from `builtin`. Note that you
can use the same argument multiple times.
You can use either `-r` / `--repo` or `-N` / `--namespace`. `-N` is there to match the
corresponding option on `spack find`.
- [x] add `-r` / `--repo` / `-N` / `--namespace` argument
- [x] add test
533adaaa6d