import/utils: spdx-string->license: Support '+' operator.

import/utils: spdx-string->license: Support '+' operator.

Previously, '+' was supported only via special cases for deprecated
GNU identifiers like 'GPL-N+'.  This commit adds support
for other uses of '+', such as 'AFL-2.0+' and 'LPPL-1.0+'.

Strictly speaking, '+' is an operator, not part of the SPDX license
identifier, but it is useful to handle it here.

* guix/import/utils.scm (spdx-string->license): Support '+' operator.
* tests/import-utils.scm ("spdx-string->license"): Test it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>