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/import/github.scm

@@ -330,6 +330,6 @@ releases."
    (name 'github)
    (description "Updater for GitHub packages")
    (pred github-package?)
-   (latest latest-release)))
+   (import latest-release)))