guix package: Add '--export-channels'.

* guix/channels.scm (sexp->channel): Export.
* guix/describe.scm: Use (guix channels).
(manifest-entry-provenance): New procedure.
* guix/scripts/package.scm (channel=?, export-channels): New
procedures.
(show-help, %options): Add '--export-channels'.
(process-query): Honor it.
* build-aux/build-self.scm (build-program)[select?]: Exclude (guix
channels) to account for the (guix describe) change above.
* doc/guix.texi (Invoking guix package): Document it.