refresh: Warn about packages that lack an updater.

refresh: Warn about packages that lack an updater.

* guix/upstream.scm (package-update-path): Rename to...
(package-latest-release): ... this.  Remove 'version>?' check.
(package-latest-release*): New procedure.
(package-update): Use it.
* guix/scripts/refresh.scm (lookup-updater): Rename to...
(lookup-updater-by-name): ... this.
(warn-no-updater): New procedure.
(update-package): Add #:warn? parameter and honor it.
(check-for-package-update): New procedure.
(guix-refresh)[warn?]: New variable.
Replace inline code when UPDATE? is false with a call to
'check-for-package-update'.
Pass WARN? to 'check-for-package-update' and 'update-package'.
* doc/guix.texi (Invoking guix refresh): Document it.  Fix a couple of
typos.