import: gem: Add recursive import.
* doc/guix.texi (Invoking guix import): Document gem recursive import.
* guix/import/gem.scm (gem->guix-package): Return package and dependencies
values.
(gem-recursive-import): New procedure.
* guix/scripts/import/gem.scm (show-help, %options): Add recursive option.
(guix-import-gem): Use 'gem-recursive-import'.
* tests/gem.scm (test-json): Rename to 'test-foo-json'.
("gem->guix-package"): Use 'test-foo-json'.
(test-bar-json, test-bundler-json): New variables.
("gem-recursive-import"): New test.
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