Improve the output of `guix refresh -l`

[Zhu Zihao]

Hello, Guix!

Inspired by
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=209a3274f8702acd32fa2a489667048ca4ad304b.
I found that the output of `guix refresh -l` for package have many
dependants is not convenient to read.

for example, run `guix refresh -l pango`, The terminal prints:

```
Building the following 2945 packages would ensure 7521 dependent
packages are rebuilt: ......<long long package list>
```

The package list is too long to read, flood my terminal with package
names. It will hide the actual rebuilt count, I have to pipe the output
to less command to make sure I can see it.

What about add an option like `--detail` for `guix refresh -l`?
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Zihao

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