channels: Don't pull from the same channel more than once.

Previous 'channel-instance->manifest' would call
'latest-channel-derivation', which could trigger another round of
'latest-repository-commit' for no good reason.

* guix/channels.scm (resolve-dependencies): New procedure.
(channel-instance-derivations)[edges]: New variable.
[instance->derivation]: New procedure.
* tests/channels.scm (make-instance): Use 'checkout->channel-instance'
instead of 'channel-instance'.
("channel-instances->manifest"): New test.