Complete package names for more commands

AFAICT package names currently are completed only for the update command. I think it would be useful for:

- `show` / `info`
- `prohibits` / `why-not`
- `depends`/ `why`

And maybe some more. Should I provide a PR or will you make a quick update yourself?

PS! Thx for the excellent work.

Start by locating the existing package-name completion for the update command and compare its behavior with the command entry points for show/info, prohibits/why-not, and depends/why. Confirm which commands should support package completion, then verify that each listed command suggests the expected package names; the issue does not name files or tests.

Written by the indexing model from the issue text.