upstream-updater: Rename record field.

The next commits will make the functions, which are currently importing the
latest version of a package, change into importing the latest or a given
version of the package (for those updaters supporting specifying a version).
Thus the name ‘latest‘ is no longer appropriate.

* guix/upstream.scm (upstream-updater) Rename field [latest] to
  [import]. (lookup-updater, package-latest-release) Adjust fieldname
  accordingly.
* guix/gnu-maintenance.scm (%gnu-updater, %gnu-ftp-updater,
  %savannah-updater, %sourceforge-updater, %xorg-updater,
  %kernel.org-updater, %generic-html-updater),
  guix/import/cpan.scm (%cpan-updater),
  guix/import/cran.scm (%cran-updater, %bioconductor-updater),
  guix/import/crate.scm (%crate-updater),
  guix/import/egg.scm (%egg-updater),
  guix/import/elpa.scm (%elpa-updater),
  guix/import/gem.scm (%gem-updater),
  guix/import/git.scm (%generic-git-updater),
  guix/import/github.scm (%github-updater),
  guix/import/gnome.scm (%gnome-updater),
  guix/import/hackage.scm (%hackage-updater),
  guix/import/hexpm.scm (%hexpm-updater),
  guix/import/kde.scm (%kde-updater),
  guix/import/launchpad.scm (%launchpad-updater),
  guix/import/minetest.scm (%minetest-updater),
  guix/import/opam.scm (%opam-updater),
  guix/import/pypi.scm (%pypi-updater),
  guix/import/stackage.scm (%stackage-updater),
  tests/import-github.scm (found-sexp)
  tests/transformations.scm ("options->transformation, with-latest"):
  Adjust fieldname accordingly.

guix/upstream.scm

@@ -66,7 +66,7 @@
             upstream-updater-name
             upstream-updater-description
             upstream-updater-predicate
-            upstream-updater-latest
+            upstream-updater-import
 
             upstream-input-change?
             upstream-input-change-name
@@ -241,7 +241,7 @@ correspond to the same version."
   (name        upstream-updater-name)
   (description upstream-updater-description)
   (pred        upstream-updater-predicate)
-  (latest      upstream-updater-latest))
+  (import      upstream-updater-import))
 
 (define (importer-modules)
   "Return the list of importer modules."
@@ -272,7 +272,7 @@ correspond to the same version."
   "Return an updater among UPDATERS that matches PACKAGE, or #f if none of
 them matches."
   (find (match-lambda
-          (($ <upstream-updater> name description pred latest)
+          (($ <upstream-updater> name description pred import)
            (pred package)))
         updaters))
 
@@ -285,9 +285,9 @@ them until one of them returns an upstream source.  It is the caller's
 responsibility to ensure that the returned source is newer than the current
 one."
   (any (match-lambda
-         (($ <upstream-updater> name description pred latest)
+         (($ <upstream-updater> name description pred import)
           (and (pred package)
-               (latest package))))
+               (import package))))
        updaters))
 
 (define* (package-latest-release* package