bug#44928: guix upgrade command does not accept a list of packages

[Luis Felipe]

Guix documentation says that `guix upgrade` is an alias for `guix package -u`, 
but the latter can upgrade a list of packages, while the former fails.


## Steps to reproduce

1. guix pull
2. guix upgrade --dry-run
3. Pick two packages from the list shown in (2), say, packages A and B
4. guix upgrade A B


## Expected result

Packages A and B are upgraded.


## Unexpected result

Step 4 fails with the following message:

    guix upgrade: error: B: extraneous argument


## System information

Using guix 0ae0efd with the Guix System.


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Luis Felipe López Acevedo
https://luis-felipe.gitlab.io/