gnu-maintenance: Generalize, leading to (guix upstream).

gnu-maintenance: Generalize, leading to (guix upstream).

* guix/gnu-maintenance.scm (<gnu-release>): Remove.
  (coalesce-releases): Move to upstream.scm.  Rename to
  'coalesce-sources'; adjust callers.
  (releases, latest-release): Return <upstream-source> objects instead
  of <gnu-release> objects.
  (latest-release*, non-emacs-gnu-package?): New procedures.
  (gnu-release-archive-types): Remove.
  (%gnu-updater): New variable.
  (package-update-path, download-tarball, package-update,
  update-package-source): Move to...
* guix/upstream.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.
* po/guix/POTFILES.in: Replace gnu-maintenance.scm with upstream.scm.
* guix/scripts/refresh.scm (%updaters): New variable.
  (update-package): Adjust to new 'package-update' interface.
  (guix-refresh): Adjust to new 'package-update-path'.  Remove
  'false-if-exception' around it.