Previously, the resulting 'signature-urls' would contain N times the
same URL.
* guix/upstream.scm (coalesce-sources): Fix TWO in 'signature-urls'.
* tests/upstream.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
* guix/gnu-maintenance.scm (latest-ftp-release): Add #:file->signature
parameter. Honor it.
(latest-gnome-release): Pass #:file->signature.
* guix/upstream.scm (coalesce-sources): Keep 'signature-urls' as #f
unless both sources provide it.
This is a followup to 0a7c5a0.
* guix/import/gnu.scm (preferred-archive-type): Use
'upstream-source-archive-types' instead of 'gnu-release-archive-types'.
(gnu-package->sexp): Use 'upstream-source-urls' et al. Update call to
'download-tarball'.
(gnu->guix-package): Use <upstream-source> instead of <gnu-release>.
* guix/upstream.scm (upstream-source-archive-types): Export.
* 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.
