`login` command should validate channel names as full channel names
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Current evaluation
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Issue body
### What needs to get done
Charmhub doesn't validate channel names when requesting permissions, but in the end needs the channel name to be the full name (e.g. `latest/edge` rather than `edge`). `login` should validate this for the user.
### Why it needs to get done
As a complement to https://github.com/canonical/charmcraft/issues/1657
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