maint: Switch to Guile-JSON 3.x.

Guile-JSON 3.x is incompatible with Guile-JSON 1.x, which we relied on
until now: it maps JSON dictionaries to alists (instead of hash tables),
and JSON arrays to vectors (instead of lists).  This commit is about
adjusting all the existing code to this new mapping.

* m4/guix.m4 (GUIX_CHECK_GUILE_JSON): New macro.
* configure.ac: Use it.
* doc/guix.texi (Requirements): Mention the Guile-JSON version.
* guix/git-download.scm (git-fetch)[guile-json]: Use GUILE-JSON-3.
* guix/import/cpan.scm (string->license): Expect vectors instead of
lists.
(module->dist-name): Use 'json-fetch' instead of 'json-fetch-alist'.
(cpan-fetch): Likewise.
* guix/import/crate.scm (crate-fetch): Likewise, and call 'vector->list'
for DEPS.
* guix/import/gem.scm (rubygems-fetch): Likewise.
* guix/import/json.scm (json-fetch-alist): Remove.
* guix/import/pypi.scm (pypi-fetch): Use 'json-fetch' instead of
'json-fetch-alist'.
(latest-source-release, latest-wheel-release): Call 'vector->list' on
RELEASES.
* guix/import/stackage.scm (stackage-lts-info-fetch): Use 'json-fetch'
instead of 'json-fetch-alist'.
(lts-package-version): Use 'vector->list'.
* guix/import/utils.scm (hash-table->alist): Remove.
(alist->package): Pass 'vector->list' on the inputs fields, and default
to the empty vector.
* guix/scripts/import/json.scm (guix-import-json): Remove call to
'hash-table->alist'.
* guix/swh.scm (define-json-reader): Expect pair? or null? instead of
hash-table?.
[extract-field]: Use 'assoc-ref' instead of 'hash-ref'.
(json->branches): Use 'map' instead of 'hash-map->list'.
(json->checksums): Likewise.
(json->directory-entries, origin-visits): Call 'vector->list' on the
result of 'json->scm'.
* tests/import-utils.scm ("alist->package with dependencies"): New test.
* gnu/installer.scm (build-compiled-file)[builder]: Use GUILE-JSON-3.
* gnu/installer.scm (installer-program)[installer-builder]: Likewise.
* gnu/installer/locale.scm (iso639->iso639-languages): Use 'assoc-ref'
instead of 'hash-ref', and pass vectors through 'vector->list'.
(iso3166->iso3166-territories): Likewise.
* gnu/system/vm.scm (system-docker-image)[build]: Use GUILE-JSON-3.
* guix/docker.scm (manifest, config): Adjust for Guile-JSON 3.
* guix/scripts/pack.scm (docker-image)[build]: Use GUILE-JSON-3.
* guix/import/github.scm (fetch-releases-or-tags): Update docstring.
(latest-released-version): Use 'assoc-ref' instead of 'hash-ref'.  Pass
the result of 'fetch-releases-or-tags' to 'vector->list'.
* guix/import/launchpad.scm (latest-released-version): Likewise.