Suggested by Ricardo Wurmus and Simon Tournier.
* guix/channels.scm (sexp->channel-introduction): New procedure.
(read-channel-metadata): Use it.
(profile-channels)[sexp->channel-introduction]: Remove.
* tests/channels.scm ("latest-channel-instances, authenticate dependency"):
New test.
* doc/guix.texi (Channels)[Declaring Channel Dependencies]: Augment example.